Light-Emitting Element, Light-Emitting Device, Electronic Device, and Lighting Device

ABSTRACT

A light-emitting element of the present invention can have sufficiently high emission efficiency with a structure including a host material being able to remain chemically stable even if a phosphorescent compound having higher emission energy is used as a guest material. The relation between the relative emission intensity and the emission time of light emission obtained from the host material and the guest material contained in a light-emitting layer is represented by a multicomponent decay curve. The relative emission intensity of the slowest component of the multicomponent decay curve becomes 1/100 for a short time within a range where the slowest component is not interfered with by quenching of the host material (the emission time of the slowest component is preferably less than or equal to 15 μsec); thus, sufficiently high emission efficiency can be obtained.

This application is a continuation of copending U.S. application Ser.No. 14/150,388, filed on Jan. 8, 2014 which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to an object, a method, a manufacturingmethod, a process, a machine, manufacture, or a composition of matter.In particular, the present invention relates to, for example, asemiconductor device, a display device, a light-emitting device, a powerstorage device, a driving method thereof, or a manufacturing methodthereof Specifically, one embodiment of the present invention relates toa light-emitting element in which an organic compound capable ofproviding light emission by application of an electric field is providedbetween a pair of electrodes, and also relates to a light-emittingdevice, an electronic device, and a lighting device including such alight-emitting element.

BACKGROUND ART

A light-emitting element using an organic compound as a luminous body,which has features such as thinness, lightness, high-speed response, andDC drive at low voltage, is expected to be applied to a next-generationflat panel display. In particular, a display device in whichlight-emitting elements are arranged in a matrix is considered to haveadvantages in a wide viewing angle and excellent visibility over aconventional liquid crystal display device.

A light-emitting element is said to have the following light emissionmechanism: when voltage is applied between a pair of electrodes with anEL layer containing a light-emitting substance provided therebetween,electrons injected from a cathode and holes injected from an anode areexcited in a light emission center of the EL layer, and energy isreleased and light is emitted when the excited state returns to a groundstate. There can be two types of the excited states generated in thecase of using an organic compound as a light-emitting substance: asinglet excited state and a triplet excited state. Luminescence from thesinglet excited state (S1) is referred to as fluorescence, andluminescence from the triplet excited state (T1) is referred to asphosphorescence. The statistical generation ratio of the excited statesin the light-emitting element is considered to be S1:T1=1:3.

Development for improving element characteristics has been conducted;for example, a light-emitting element having a structure utilizing notonly fluorescence but also phosphorescence has been developed. In alight-emitting layer of the light-emitting element, a host material anda guest material are contained, and a phosphorescent material exhibitinghigh energy emission is used as the guest material (e.g., see PatentDocument 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2010-182699

DISCLOSURE OF INVENTION

In general, it is thought that to improve the emission efficiency of alight-emitting element using a host material and a guest material, theT1 level (the level in the triplet excited state) of the host materialis preferably higher than that of the guest material. However, in thecase where a phosphorescent compound having high emission energy (e.g.,a blue phosphorescent compound) is used as a guest material, the T1level of a host material needs to be higher than that in the case wherea phosphorescent compound having lower emission energy (e.g., a green orred phosphorescent compound) is used as a guest material; thus, there isa problem in that the host material becomes chemically unstable.

An object of one embodiment of the present invention is to provide achemically stable light-emitting device. Another object of oneembodiment of the present invention is to provide a light-emittingdevice having high emission efficiency. Another object of one embodimentof the present invention is to provide a highly reliable light-emittingdevice. Another object of one embodiment of the present invention is toprovide a light-emitting device in which image burn-in is unlikely tooccur. Another object of one embodiment of the present invention is toprovide a light-emitting device in which delayed light emission isperformed. Another object of one embodiment of the present invention isto provide a novel light-emitting device. Another object of oneembodiment of the present invention is to provide an excellentlight-emitting device.

Note that the description of these objects does not disturb theexistence of other objects. Note that in one embodiment of the presentinvention, there is no need to achieve all of the objects. Other objectswill be apparent from the description of the specification, thedrawings, the claims, and the like and other objects can be derived fromthe description of the specification, the drawings, the claims, and thelike.

In view of the above background, a light-emitting element of oneembodiment of the present invention can have sufficiently high emissionefficiency with a structure including a host material being able toremain chemically stable even if a phosphorescent compound having higheremission energy is used as a guest material. The structure is asfollows: a light-emitting layer in the light-emitting element containsat least a host material and a guest material; the relation between therelative emission intensity and the emission time of light emissionobtained from these materials (e.g., photoluminesence (PL) byphotoexcitation or electroluminescence (EL) by electric fieldexcitation) at the exciton concentration in a range where concentrationquenching does not occur is represented by a multicomponent decay curve;it is preferable that the relative emission intensity (=E(t)/E₀) of theslowest component of the decay curve become 1/100 for a short timewithin a range where the slowest component is not interfered with byquenching of the host material; that is, the emission time of theslowest component is less than or equal to 15 μsec, preferably less thanor equal to 10 μsec, more preferably less than or equal to 5 μsec.

Note that the multicomponent decay curve is expressed by Formula 1below.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 1} \rbrack \mspace{585mu}} & \; \\{{{E(t)}/E_{0}} = {\sum\limits_{i = 1}^{n}{A_{i}{\exp ( {{- t}/\tau_{i}} )}}}} & (1)\end{matrix}$

(Note that E₀ indicates an initial emission intensity, E(t) indicates anemission intensity at time (t), A is a constant, τ indicates a lifetime,and n indicates the number of components of a decay curve.)

Under the above conditions, even when the T1 level of a host material islower than the T1 level of a guest material, energy transfer from thehost material to the guest material is possible. Since the T1 level ofthe host material is not necessarily higher than that of the guestmaterial, a chemically stable material can be used as the host material.

Accordingly, one embodiment of the present invention is a light-emittingelement including a light-emitting layer containing at least a hostmaterial and a guest material. In the light-emitting layer that has beenirradiated with a pulsed laser (the output level is set not to causeconcentration quenching), relation between the relative emissionintensity and the emission time is represented by a multicomponent decaycurve, and the emission time it takes for the relative emissionintensity of the slowest component of the decay curve to become 1/100 isless than or equal to 15 μsec, preferably less than or equal to 10 μsec,more preferably less than or equal to 5 μsec.

Another embodiment of the present invention is a light-emitting elementincluding at least a light-emitting layer between a pair of electrodes.The light-emitting layer contains two or more kinds of organiccompounds. Two or more components that show the relation between therelative emission intensity and the emission time at the time of lightemission are observed when the relative emission intensity becomes1/100. The time it takes for the relative emission intensity of theslowest component of the multicomponent decay curve to become 1/100 isless than or equal to 15 μsec, preferably less than or equal to 10 μsec,more preferably less than or equal to 5 μsec.

Another embodiment of the present invention is a light-emitting elementincluding at least a light-emitting layer between a pair of electrodes.The light-emitting layer contains at least a first organic compound (ahost material) and a second organic compound (a guest material). Thesecond organic compound is an organic metal complex. The T1 level of thefirst organic compound is lower than that of the second organiccompound. Two or more components that show the relation between therelative emission intensity and the emission time are observed when therelative emission intensity becomes 1/100. The emission time it takesfor the relative emission intensity of the slowest component of themulticomponent decay curve to become 1/100 is less than or equal to 15μsec, preferably less than or equal to 10 μsec, more preferably lessthan or equal to 5 μsec.

Note that in each of the above structures, an organic compound whose T1level is lower than that of the guest material can be used as the hostmaterial; thus, the light-emitting element can be fabricated withoutusing a chemically unstable organic compound as the host material even.

In any of the above structures, the host material is preferably selectedsuch that the guest material such that the T1 level of the host materialis lower than that of the guest material, and the difference in T1 levelis greater than or equal to 0 eV and less than or equal to 0.2 eV.Accordingly, a chemically stable host material can be used withoutdecreasing emission efficiency, leading to a long-lifetimelight-emitting element.

Other embodiments of the present invention are not only a light-emittingdevice including the light-emitting element but also an electronicdevice and a lighting device each including the light-emitting device.Accordingly, a light-emitting device in this specification refers to animage display device or a light source (including a lighting device). Inaddition, the light-emitting device includes, in its category, all of amodule in which a light-emitting device is connected to a connector suchas a flexible printed circuit (FPC) or a tape carrier package (TCP), amodule in which a printed wiring board is provided on the tip of a TCP,and a module in which an integrated circuit (IC) is directly mounted ona light-emitting element by a chip on glass (COG) method.

A light-emitting element of one embodiment of the present invention canhave high emission efficiency. A light-emitting element of oneembodiment of the present invention can have a long lifetime byincluding a chemically stable host material in a light-emitting layer. Alight-emitting device of one embodiment of the present invention canhave high reliability by including the light-emitting element. Anelectronic device and a lighting device of one embodiment of the presentinvention can have high reliability by including the light-emittingdevice.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A and 1B illustrate a concept of one embodiment of the presentinvention.

FIG. 2 illustrates a structure of a light-emitting element.

FIG. 3 illustrates a structure of a light-emitting element.

FIGS. 4A and 4B illustrate structures of light-emitting elements.

FIGS. 5A and 5B illustrate a light-emitting device.

FIGS. 6A to 6D illustrate electronic devices.

FIGS. 7A to 7C illustrate an electronic device.

FIG. 8 illustrates lighting devices.

FIG. 9 illustrates a structure of a light-emitting element.

FIG. 10 shows current density versus luminance characteristics of alight-emitting element 1 and a comparative light-emitting element 2.

FIG. 11 shows voltage versus luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 12 shows luminance versus current efficiency characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 13 shows voltage versus current characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.

FIG. 14 shows an emission spectrum of the light-emitting element 1.

FIG. 15 shows reliability of each of the light-emitting element 1 andthe comparative light-emitting element 2.

FIG. 16 shows phosphorescence spectra of the light-emitting elements.

FIG. 17 shows emission times of the light-emitting elements.

FIGS. 18A and 18B illustrate a light-emitting device of one embodimentof the present invention.

FIGS. 19A and 19B each illustrate a light-emitting device of oneembodiment of the present invention.

FIGS. 20A to 20E each illustrate a lighting device of one embodiment ofthe present invention.

FIGS. 21A and 21B illustrate a touch sensor of one embodiment of thepresent invention.

FIG. 22 is a circuit diagram of a touch sensor of one embodiment of thepresent invention.

FIG. 23 is a cross-sectional view of a touch sensor of one embodiment ofthe present invention.

FIG. 24 illustrates a module using a light-emitting device of oneembodiment of the present invention.

FIGS. 25A and 25B each illustrate a light-emitting element of oneembodiment of the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description, and modes and details thereof canbe modified in various ways without departing from the spirit and thescope of the present invention. Therefore, the present invention shouldnot be construed as being limited to the description in the followingembodiments.

Embodiment 1

In this embodiment, described are a concept and a specific structure ofa light-emitting element of one embodiment of the present invention. Thelight-emitting element includes a host material capable of remainingchemically stable even if a phosphorescent compound having high emissionenergy is used as a guest material.

In a light-emitting element of one embodiment of the present invention,a light-emitting layer is provided between a pair of electrodes, and thelight-emitting layer contains at least a host material and a guestmaterial (the exciton concentration is in a range where concentrationquenching does not occur). The relation between the relative emissionintensity and the emission time of light emission obtained from thesematerials (e.g., photoluminesence (PL) by photoexcitation orelectroluminescence (EL) by electric field excitation) is represented bya multicomponent decay curve. The relative emission intensity of theslowest component of the decay curve becomes 1/100 for a short timewithin a range where the slowest component is not interfered with byquenching of the host material (preferably less than or equal to 15μsec); thus, sufficiently high emission efficiency can be obtained.

At this time, energy transfer is possible even in the case where the T1level of the host material is lower than that of the guest material.Since the T1 level of the host material is not necessarily higher thanthat of the guest material, a chemically stable material can be used asthe host material. Accordingly, in one embodiment of the presentinvention, a host material whose T1 level is lower than that of a guestmaterial can be used.

A structure of these materials in one embodiment of the presentinvention is described with reference to FIGS. 1A and 1B.

FIG. 1A illustrates relation between an energy state of excitons in ahost material 11 and an energy state of excitons in a guest material 12.A light-emitting layer contains at least the host material 11 and theguest material 12. The triplet excited state of the guest material 12 isa T1(g) level, and an exciton 10 generated from the guest material 12 islocated at this level. The triplet excited state of the host material 11is a T1(h) level that is lower than the T1(g) level of the guestmaterial 12 by ΔE(eV) energy.

In this case, an excitation energy at the T1(g) level of the guestmaterial 12 transfers (Y_(g)) to the T1(h) level of the host material 11at a rate of [D₀*]×K₂. Note that [D₀*] represents the concentration ofexcitons in the guest material, and K₂ represents a rate constant ofexcitation energy transfer from the guest material 12 to the hostmaterial 11. Furthermore, excitation energy can transfer (Y_(b)) fromthe T1(h) level of the host material 11 to the T1(g) level of the guestmaterial 12 at a rate of [H₀*]×K₃. Note that [H₀*] represents theconcentration of excitons in the host material, and K₃ represents a rateconstant of excitation energy transfer from the host material 11 to theguest material 12. This physically disadvantageous energy transfer fromthe low level to the high level (hereinafter, referred to as reverseenergy transfer) can occur because excitons are activated by energy atroom temperature. However, just after photoexcitation or electricalexcitation, the rate of excitation energy transfer from the T1(g) levelto the T1(h) level is extremely higher than the rate of reverse energytransfer from the T1(h) level to the T1(g) level. Therefore, it can beregarded that reverse energy transfer from the host material 11 to theguest material 12 hardly occurs. Note that in FIGS. 1A and 1B, K₁represents a rate constant of transfer from the T1(g) level to an S0(g)level of the guest material 12, and K₄ represents a rate constant oftransfer from the T1(h) level to an S0(h) level of the host material 11.

However, when the excitation energy transfer (Y_(g)) from the T1(g)level to the T1(h) level proceeds, the concentration of excitons at theT1(h) level increases as illustrated in FIG. 1B, so that the excitationenergy transfer (Y_(h)) from the T1(h) level to the T1(g) level occurseffectively. At this time, for effective reverse energy transfer, it isimportant that an energy difference (ΔE) between the T1(g) level and theT1(h) level is not so large. Here, combination of the host material 11and the guest material 12 which satisfies the formula 0<ΔE<0.2 eV ispreferable.

When the above-mentioned excitation energy transfers occur, radiativetransition (X_(g)) from the T1(g) level to the S0(g) level of the guestmaterial 12 and non-radiative transition (X_(h)) from the T1(h) level tothe S0(h) level of the host material 11 also occur at the same time.Note that in FIGS. 1A and 1B, K₁ represents a transition rate constantfrom the T1(g) level to the S0(g) level of the guest material 12, and K₄represents a transition rate constant from the T1(h) level to the S0(h)level of the host material 11. At this time, it is also important forhigh efficiency light emission that the rate of the non-radiativetransition (X_(h)) be much lower than that of the radiative transition(X_(g)). It is preferable that the radiative transition (X_(g)) befaster than 0.2 (μsec)⁻¹, and the non-radiative transition (X_(h)) beslower than 10 (msec)⁻¹.

That is, the rate of reverse energy transfer is made sufficiently higherthan that of non-radiative transition of the host material, and the rateof radiative transition of the guest material is made sufficientlyhigher than that of non-radiative transition of the host material,whereby high efficiency light emission can be obtained.

As described above, a light-emitting element of one embodiment of thepresent invention also utilizes energy that reversely transfers from alow level for its light emission, and thus has a feature in that a curverepresenting an emission time obtained by PL measurement is amulticomponent decay curve. The relative emission intensity of theslowest component of the decay curve becomes 1/100 for a short timewithin a range where the slowest component is not interfered with byquenching of the host material, that is, the emission time of theslowest component is less than or equal to 15 μsec, preferably less thanor equal to 10 μsec, more preferably less than or equal to 5 μsec; thus,sufficiently high emission efficiency can be obtained.

Note that in addition to the above-mentioned state, a measurement resultmay show a multicomponent decay curve in a state where the power densityof a pulsed laser is set high and the exciton concentration is high.This is because the exciton concentration becomes high and interactionamong excitons leads to the triplet-triplet extinction. This phenomenonis called concentration quenching. The measurement needs to be performedin a state where the power density of a pulsed laser is set low and theexciton concentration is low to avoid influence of concentrationquenching.

Next, a structure of a light-emitting element of one embodiment of thepresent invention is described with reference to FIG. 2.

As illustrated in FIG. 2, the light-emitting element of one embodimentof the present invention has a structure in which a light-emitting layer104 containing a first organic compound and a second organic compound isprovided between a pair of electrodes (an anode 101 and a cathode 102).The light-emitting layer 104 is one of functional layers included in anEL layer 103 that is in contact with the pair of electrodes. The ELlayer 103 can include, in addition to the light-emitting layer 104, anyof a hole-injection layer, a hole-transport layer, an electron-transportlayer, an electron-injection layer, and the like as appropriate atdesired positions. Note that the light-emitting layer 104 contains atleast a first organic compound 105 serving as a host material and asecond organic compound 106 serving as a guest material.

A material having an excellent hole-transport property or a materialhaving an excellent electron-transport property can be used as the firstorganic compound 105 serving as a host material.

Examples of the material having an excellent hole-transport propertythat can be used as the first organic compound 105 include aromaticamine compounds such as4-(1-naphthyl)-4′-phenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBiNB),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1),4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA),2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPA2SF),N,N′-bis(9-phenylcarbazol-3-yl)-N,N-diphenylbenzene-1,3-diamine(abbreviation: PCA2B),N-(9,9-dimethyl-2-diphenylamino-9H-fluoren-7-yl)diphenylamine(abbreviation: DPNF),N,N′,N″-triphenyl-N,N′,N″-tris(9-phenylcarbazol-3-yl)benzene-1,3,5-triamine(abbreviation: PCA3B),2-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: PCASF),2-[N-(4-diphenylaminophenyl)-N-phenylamino]spiro-9,9′-bifluorene(abbreviation: DPASF),N,N′-bis[4-(carbazol-9-yl)phenyl]-N,N′-diphenyl-9,9-dimethylfluorene-2,7-diamine(abbreviation: YGA2F),4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB),N-(9,9-dimethyl-9H-fluoren-2-yl)-N-{9,9-dimethyl-2-[N′-phenyl-N-(9,9-dimethyl-9H-fluoren-2-yl)amino]-9H-fluoren-7-yl}phenylamine(abbreviation: DFLADFL), 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB or α-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3-[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA1),3,6-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzDPA2),4,4′-bis(N-{4-[N′-(3-methylphenyl)-N-phenylamino]phenyl}-N-phenylamino)biphenyl(abbreviation: DNTPD),3,6-bis[N-(4-diphenylaminophenyl)-N-(1-naphthyl)amino]-9-phenylcarbazole(abbreviation: PCzTPN2), and3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2). In addition, the following compounds includinga carbazole skeleton can be used, for example:4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB), and9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA).The substances mentioned here are mainly ones that have a hole mobilityof 10⁻⁶ cm²/Vs or higher. Note that any substance other than the abovesubstances may be used as long as it has a hole-transport property.

Examples of the material having an excellent electron-transport propertythat can be used as the first organic compound 105 include thefollowings: heterocyclic compounds having polyazole skeletons, such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenyl)-1,2,4-triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), and2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); heterocyclic compounds having quinoxalineskeletons or dibenzoquinoxa line skeletons, such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II), 7-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline(abbreviation: 7mDBTPDBq-II),6-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:6mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II), and2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq); heterocyclic compounds having diazineskeletons (pyrimidine skeletons or pyrazine skeletons), such as4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine (abbreviation:4,6mPnP2Pm), 4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine(abbreviation: 4,6mDBTP2Pm-II), and4,6-bis[3-(9H-carbazol-9-yl)phenyl]pyrimidine (abbreviation:4,6mCzP2Pm); and heterocyclic compounds having pyridine skeletons, suchas 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation: 35DCzPPy),1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation: TmPyPB), and3,3′,5,5′-tetra[(m-pyridyl)-phen-3-yl]biphenyl (abbreviation: BP4mPy).Among the above-described compounds, the heterocyclic compounds havingquinoxaline skeletons or dibenzoquinoxaline skeletons, the heterocycliccompounds having diazine skeletons, and the heterocyclic compoundshaving pyridine skeletons have high reliability and are thus preferable.Other examples of the material having an excellent electron-transportproperty include the followings: triaryl phosphine oxides, such asphenyl-di(1-pyrenyl)phosphine oxide (abbreviation: POPy₂),spiro-9,9′-bifluoren-2-yl-diphenylphosphine oxide (abbreviation: SPPO1),2,8-bis(diphenylphosphoryl)dibenzo[b,d]thiophene (abbreviation: PPT),and 3-(diphenylphosphoryl)-9-[4-(diphenylphosphoryl)phenyl]-9H-carbazole(abbreviation: PPO21); and triaryl borane such astris[2,4,6-trimethyl-3-(3-pyridyl)phenyl]borane (abbreviation: 3TPYMB).The substances mentioned here have an electron-transport property andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any substance other than the above substances may be used aslong as it has an electron-transport property.

Note that the light-emitting layer may contain a third organic compoundin addition to the first organic compound (the host material) and thesecond organic compound (the guest material). To obtain high emissionefficiency by adjustment of a balance between holes and electrons in thelight-emitting layer, when the first organic compound has ahole-transport property, the third organic compound preferably has anelectron-transport property. In contrast, when the first organiccompound has an electron-transport property, the third organic compoundpreferably has a hole-transport property. In either case, the T1(h)level of the first organic compound is preferably lower than the T1(g)level of the second organic compound. Note that the T1 level of thethird organic compound may be higher than the T1(g) level. This isbecause energy at the T1 level of the third organic compound is rapidlycollected to energy at the T1(h) level (located at lower than the T1level of the third organic compound) of the first organic compound.

As the second organic compound 106 serving as a guest material, anorganic metal complex (a phosphorescent compound) that is alight-emitting substance converting triplet excitation energy into lightemission can be used, for example.

Examples of the material that can be used as the second organic compound106 include bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III)tetrakis(1-pyrazolyl)borate (abbreviation: FIr6),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)]iridium(III) picolinate(abbreviation: FIrpic),bis[2-(3′,5′-bistrifluoromethylphenyl)pyridinato-N,C^(2′)]iridium(III)picolinate(abbreviation: [Ir(CF₃ppy)₂(pic)]),bis[2-(4′,6′-difluorophenyl)pyridinato-N,C^(2′)iridium(III)acetylacetonate (abbreviation: FIracac),tris(2-phenylpyridinato)iridium(III) (abbreviation: [Ir(ppy)₃]),bis(2-phenylpyridinato)iridium(III) acetylacetonate (abbreviation:[Ir(ppy)₂(acac)]), bis(benzo[h]quinolinato)iridium(III) acetylacetonate(abbreviation: [Ir(bzq)₂(acac)]), bis(2,4-diphenyl-1,3-oxazolato-N,acetylacetonate (abbreviation: [Ir(dpo)₂(acac)]),bis{2-[4′-(perfluorophenyl)phenyl]pyridinato-N,C^(2′)]iridium(III)acetylacetonate (abbreviation: [Ir(p-PF-ph)₂(acac)]),bis(2-phenylbenzothiazolato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(bt)₂(acac)]),bis[2-(2′-benzo[4,5-α]thienyl)pyridinato-N,C^(3′)]iridium(III)acetylacetonate(abbreviation: [Ir(btp)₂(acac)]),bis(1-phenylisoquinolinato-N,C^(2′))iridium(III)acetylacetonate(abbreviation: [Ir(piq)₂(acac)]),(acetylacetonato)bis[2,3-bis(4-fluorophenyl)quinoxalinato]iridium(III)(abbreviation: [Ir(Fdpq)₂(acac)]),(acetylacetonato)bis(2,3,5-triphenylpyrazinato)iridium(III)(abbreviation: [Ir(tppr)₂(acac)]),2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphineplatinum(II)(abbreviation: PtOEP),tris(acetylacetonato)(monophenanthroline)terbium(III) (abbreviation:Tb(acac)₃(Phen)),tris(1,3-diphenyl-1,3-propanedionato)(monophenanthroline)europium(III)(abbreviation: Eu(DBM)₃(Phen)), andtris[1-(2-thenoyl)-3,3,3-trifluoroacetonato](monophenanthroline)europium(III)(abbreviation: Eu(TTA)₃(Phen)).

In the light-emitting layer of the light-emitting element described inthis embodiment, the host material and the guest material contained inthe light-emitting layer are selected to satisfy the following: therelation between the relative emission intensity and the emission timeof light emission obtained from these materials (e.g., photoluminesence(PL) by photoexcitation or electroluminescence (EL) by electric fieldexcitation) is represented by a multicomponent decay curve; the relativeemission intensity of the slowest component of the multicomponent decaycurve becomes 1/100 for a short time within a range where the slowestcomponent is not interfered with by quenching of the host material, thatis, the emission time of the slowest component is less than or equal to15 μsec, preferably less than or equal to 10 μsec, more preferably lessthan or equal to 5 μsec.

In the light-emitting element having the above feature, energy transferis possible even in the case where the T1 level of the host material islower than that of the guest material. Since the T1 level of the hostmaterial is not necessarily higher than that of the guest material, achemically stable material can be used as the host material.

Accordingly, in this embodiment, a chemically stable material can beused as a host material in a light-emitting layer of a light-emittingelement; thus, the light-emitting element can have a long lifetime. Inthe structure of this embodiment, when the T1 level of the host materialis lower than that of the guest material, delayed light emissionassociated with reverse energy transfer occurs. Since a host material inthe T1 level is non-radiative at room temperature, it is concerned thata light-emitting layer exhibiting delayed light emission has lowefficiency. However, in the above range, the rate of reverse energytransfer and the rate of radiative transition of the guest material aresufficiently higher than the rate of non-radiative transition of thehost material (a radiationless deactivation rate of the host material);thus, element characteristics are not affected and a light-emittingelement having high emission efficiency can be obtained.

Note that in this embodiment, the example in which the relation betweenthe relative emission intensity and the emission time is represented bya multicomponent decay curve is described, but one embodiment of thepresent invention is not limited thereto. Depending on circumstances orconditions, the relation between the relative emission intensity and theemission time of one embodiment of the present invention is notrepresented by a multicomponent decay curve in some cases.

Embodiment 2

In this embodiment, an example of a light-emitting element of oneembodiment of the present invention is described with reference to FIG.3.

In the light-emitting element described in this embodiment, asillustrated in FIG. 3, an EL layer 203 including a light-emitting layer206 is provided between a pair of electrodes (a first electrode (anode)201 and a second electrode (cathode) 202), and the EL layer 203 includesa hole-injection layer 204, a hole-transport layer 205, anelectron-transport layer 207, an electron-injection layer 208, and thelike in addition to the light-emitting layer 206.

As in the light-emitting element described in Embodiment 1, thelight-emitting layer 206 contains at least the first organic compound209 serving as a host material and the second organic compound 210serving as a guest material. Since the same substances described inEmbodiment 1 can be used as the first organic compound 209 and thesecond organic compound 210, and description thereof is omitted.

In addition to the first organic compound 209 serving as a host materialand the second organic compound 210 serving as a guest material, thelight-emitting layer 206 may also contain the third organic compoundhaving a property opposite to the property of the first organic compound209 (a hole-transport property or an electron-transport property).

Next, a specific example in manufacturing the light-emitting elementdescribed in this embodiment is described.

For the first electrode (anode) 201 and the second electrode (cathode)202, a metal, an alloy, an electrically conductive compound, a mixturethereof, or the like can be used. Specifically, indium oxide-tin oxide(ITO: indium tin oxide), indium oxide-tin oxide containing silicon orsilicon oxide, indium oxide-zinc oxide (indium zinc oxide), indium oxidecontaining tungsten oxide and zinc oxide, gold (Au), platinum (Pt),nickel (Ni), tungsten (W), chromium (Cr), molybdenum (Mo), iron (Fe),cobalt (Co), copper (Cu), palladium (Pd), or titanium (Ti) can be used.In addition, an element belonging to Group 1 or Group 2 of the periodictable, for example, an alkali metal such as lithium (Li) or cesium (Cs),an alkaline earth metal such as magnesium (Mg), calcium (Ca), orstrontium (Sr), an alloy containing such an element (e.g., MgAg orAlLi), a rare earth metal such as europium (Eu) or ytterbium (Yb), analloy containing such an element, or graphene can be used. The firstelectrode (anode) 201 and the second electrode (cathode) 202 can beformed by, for example, a sputtering method or an evaporation method(including a vacuum evaporation method).

Examples of a material having an excellent hole-transport property thatcan be used for the hole-injection layer 204 and the hole-transportlayer 205 include aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB orα-NPD),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(carbazol-9-yl)triphenylamine(abbreviation: TCTA), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA), and4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB);3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2), and3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1). Alternatively, the following carbazolederivatives can be used: 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),and 9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-Carbazole (abbreviation:CzPA). The substances mentioned here are mainly materials having a holemobility of 10⁻⁶ cm²/Vs or higher. Note that substances other than theabove substances may be used as long as the hole-transport property ishigher than the electron-transport property.

Alternatively, a high molecular compound such as poly(N-vinylcarbazole)(abbreviation: PVK), poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine] (abbreviation:Poly-TPD) can be used.

As examples of an acceptor substance that is used for the hole-injectionlayer 204, a transition metal oxide or an oxide of a metal belonging toany of Group 4 to Group 8 of the periodic table can be given.Specifically, molybdenum oxide is particularly preferable.

Note that for the hole-transport layer 205 in contact with thelight-emitting layer 206, a compound similar to the organic compoundcontained in the light-emitting layer is preferably used. With thisstructure, the hole-injection barrier between the hole transport layer205 and the light-emitting layer 206 can be reduced, which can increaseemission efficiency and reduce driving voltage. That is, alight-emitting element having a small decrease in power efficiency dueto voltage loss even in the case of emitting light with high luminancecan be obtained. A particularly preferable mode for reducing thehole-injection barrier is a structure in which the hole-transport layer205 contains the same organic compound as the light-emitting layer.

The electron-transport layer 207 is a layer containing a material havingan excellent electron-transport property. For the electron-transportlayer 207, a metal complex such as tris(8-quinolinolato)aluminum(III)(abbreviation: Alq₃), tris(4-methyl-8-quinolinolato)aluminum(III)(abbreviation: Almq₃), bis(10-hydroxybenzo[h]quinolinato)beryllium(II)(abbreviation: BeBq₂), BAlq, Zn(BOX)₂, orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc(II) (abbreviation: Zn(BTZ)₂)can be used. Further, a heteroaromatic compound such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4′-tert-butylphenyl)-4-phenyl-5-(4″-biphenyl)-1,2,4-triazole(abbreviation: TAZ),3-(4-tert-butylphenyl)-4-(4-ethylphenyl)-5-(4-biphenylyl)-1,2,4-triazole(abbreviation: p-EtTAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or4,4′-bis(5-methylbenzoxazol-2-yl)stilbene (abbreviation: BzOs) can alsobe used. A high molecular compound such as poly(2,5-pyridinediyl)(abbreviation: PPy),poly[(9,9-dihexylfluorene-2,7-diyl)-co-(pyridine-3,5-diyl)](abbreviation: PF-Py), orpoly[(9,9-dioctylfluorene-2,7-diyl)-co-(2,2′-bipyridine-6,6′-diyl)](abbreviation: PF-BPy) can be used. The substances given here are mainlyones having an electron mobility of 10⁻⁶ cm²IVs or higher. Note that anysubstance other than the above substances may be used for theelectron-transport layer 207 as long as the electron-transport propertyis higher than the hole-transport property.

The electron-transport layer 207 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

The electron-injection layer 208 is a layer containing a substancehaving a high electron-injection property. For the electron-injectionlayer 208, an alkali metal, an alkaline earth metal, or a compoundthereof, such as lithium fluoride (LiF), cesium fluoride (CsF), calciumfluoride (CaF₂), or lithium oxide (LiOx), can be used. A rare earthmetal compound like erbium fluoride (ErF₃) can also be used. Any of theabove substances for forming the electron-transport layer 207 can alsobe used.

A composite material in which an organic compound and an electron donor(donor) are mixed may also be used for the electron-injection layer 208.Such a composite material is excellent in an electron-injection propertyand an electron-transport property because electrons are generated inthe organic compound by the electron donor. In this case, the organiccompound is preferably a material excellent in transporting thegenerated electrons. Specifically, for example, the above materials forforming the electron-transport layer 207 (e.g., a metal complex or aheteroaromatic compound) can be used. As the electron donor, a substanceexhibiting an electron-donating property with respect to the organiccompound may be used. Specific examples are an alkali metal, an alkalineearth metal, and a rare earth metal are preferable, and lithium, cesium,magnesium, calcium, erbium, and ytterbium. Further, an alkali metaloxide or an alkaline earth metal oxide is preferable, and for example,lithium oxide, calcium oxide, and barium oxide can be given. A Lewisbase such as magnesium oxide can also be used. An organic compound suchas tetrathiafulvalene (abbreviation: TTF) can also be used.

Note that each of the above hole-injection layer 204, hole-transportlayer 205, light-emitting layer 206, electron-transport layer 207, andelectron-injection layer 208 can be formed by, for example, anevaporation method (e.g., a vacuum evaporation method), an inkjetmethod, or a coating method.

Light emission obtained in the light-emitting layer 206 of theabove-described light-emitting element is extracted to the outsidethrough either the first electrode 201 or the second electrode 202 orboth. Therefore, either the first electrode 201 or the second electrode202 in this embodiment, or both, is an electrode having alight-transmitting property.

Note that the light-emitting element described in this embodiment is oneembodiment of the present invention and is particularly characterized bythe structure of the light-emitting layer. Therefore, when the structuredescribed in this embodiment is employed, a passive matrixlight-emitting device, an active matrix light-emitting device, and thelike can be manufactured. Each of these light-emitting devices isincluded in the present invention.

Note that there is no particular limitation on the structure of the FETin the case of manufacturing the active matrix light-emitting device.For example, a staggered FET or an inverted staggered FET can be used asappropriate. Further, a driver circuit formed over an FET substrate maybe formed using either an n-channel FET or a p-channel FET or both.Furthermore, there is no particular limitation on a semiconductormaterial used for the FET and the crystallinity of the semiconductormaterial. Examples of the semiconductor material include elementsemiconductors such as silicon, germanium, tin, selenium, and tellurium;compound semiconductors such as GaAs, GaP, InSb, ZnS, and CdS; and oxidesemiconductors such as SnO₂, ZnO, Fe₂O₃, V₂O₅, TiO₂, NiO, Cr₂O₃, Cu₂O,MnO₂, MnO, and InGaZnO (including the ones having different atomicratios). The crystallinity of the semiconductor material can be, forexample, amorphous, single crystal, polycrystalline, microcrystalline,or a mixed phase structure of these. A semiconductor material having anyof the above crystallinity can be used.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 3

In this embodiment, as one embodiment of the present invention, alight-emitting element (hereinafter referred to as tandem light-emittingelement) in which a charge generation layer is provided between aplurality of EL layers is described.

The light-emitting element described in this embodiment is a tandemlight-emitting element including a plurality of EL layers (a first ELlayer 302(1) and a second EL layer 302(2)) between a pair of electrodes(a first electrode 301 and a second electrode 304) as illustrated inFIG. 4A.

In this embodiment, the first electrode 301 functions as an anode, andthe second electrode 304 functions as a cathode. Note that the firstelectrode 301 and the second electrode 304 can have structures similarto those described in Embodiment 2. In addition, all or any of theplurality of EL layers (the first EL layer 302(1) and the second ELlayer 302(2)) may have structures similar to those described inEmbodiment 2. In other words, the structures of the first EL layer302(1) and the second EL layer 302(2) may be the same or different fromeach other and can be similar to those of the EL layers described inEmbodiment 2.

A charge generation layer 305 is provided between the plurality of ELlayers (the first EL layer 302(1) and the second EL layer 302(2)). Thecharge-generation layer 305 has a function of injecting electrons intoone of the EL layers and injecting holes into the other of the EL layerswhen voltage is applied between the first electrode 301 and the secondelectrode 304. In this embodiment, when voltage is applied such that thepotential of the first electrode 301 is higher than that of the secondelectrode 304, the charge-generation layer 305 injects electrons intothe first EL layer 302(1) and injects holes into the second EL layer302(2).

Note that for improving light extraction efficiency, thecharge-generation layer 305 preferably has a property of transmittingvisible light (specifically, the charge-generation layer 305 preferablyhas a visible light transmittance of 40% or higher). Further, thecharge-generation layer 305 functions even when it has lowerconductivity than the first electrode 301 or the second electrode 304.

The charge-generation layer 305 may have either a structure in which anelectron acceptor (acceptor) is added to an organic compound having anexcellent hole-transport property or a structure in which an electrondonor (donor) is added to an organic compound having an excellentelectron-transport property. Alternatively, both of these structures maybe stacked.

In the case of the structure in which an electron acceptor is added toan organic compound having an excellent hole-transport property, as theorganic compound having an excellent hole-transport property, forexample, an aromatic amine compound such as NPB, TPD, TDATA, MTDATA, or4,4′-bis[N-(Spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB) can be used. The substances given here are mainlyones having a hole mobility of 10⁻⁶ cm²/Vs or higher. However, anysubstance other than the above substances may be used as long thehole-transport property is higher than the electron-transport property.

Examples of the electron acceptor include a halogen compound such as7,7,8, 8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F4TCNQ) or chloranil; and a cyano compound such aspyrazino[2,3-f][1,10]phenanthroline-2,3-dicarbonitrile (abbreviation:PPDN) ordipyrazino[2,3-f:2′,3′-h]quinoxaline-2,3,6,7,10,11-hexacarbonitrile(abbreviation: HAT-CN). Examples of the electron acceptor also include atransition metal oxide, and an oxide of metals that belong to Group 4 toGroup 8 of the periodic table can be used. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferablebecause of their high electron-accepting property. Among these,molybdenum oxide is especially preferable because it is stable in theair, has a low hygroscopic property, and is easily handled.

In the case of the structure in which an electron donor is added to anorganic compound having an excellent electron-transport property, as theorganic compound having an excellent electron-transport property, forexample, a metal complex having a quinoline skeleton or a benzoquinolineskeleton, such as Alq, Almq₃, BeBq₂, or BAlq, can be used. A metalcomplex having an oxazole-based ligand or a thiazole-based ligand, suchas Zn(BOX)₂ or Zn(BTZ)₂, or the like can also be used. Other than metalcomplexes, PBD, OXD-7, TAZ, BPhen, BCP, or the like can be used. Thesubstances given here are mainly ones having an electron mobility of10⁻⁶ cm²/Vs or higher. Note that substances other than the abovesubstances may be used as long as the electron-transport property ishigher than the hole-transport property.

Further, as the electron donor, an alkali metal, an alkaline earthmetal, a rare earth metal, a metal belonging to Group 13 of the periodictable, or an oxide or carbonate thereof can be used. Specifically,lithium (Li), cesium (Cs), magnesium (Mg), calcium (Ca), ytterbium (Yb),indium (In), lithium oxide, cesium carbonate, or the like is preferablyused. An organic compound such as tetrathianaphthacene may be also usedas the electron donor.

Note that formation of the charge-generation layer 305 with use of anyof the above materials can suppress an increase in drive voltage causedby the stack of the EL layers.

Although the light-emitting element having two EL layers is described inthis embodiment, the present invention can be similarly applied to alight-emitting element in which n EL layers are stacked as illustratedin FIG. 4B. In the case where a plurality of EL layers is providedbetween a pair of electrodes as in the light-emitting element of thisembodiment, by providing the charge-generation layer between the ELlayers, the light-emitting element can emit light in a high luminanceregion while the current density is kept low. Since the current densitycan be kept low, the element can have a long lifetime. When thelight-emitting element is applied to illumination, voltage drop due toresistance of an electrode material can be reduced, thereby achievinghomogeneous light emission in a large area. In addition, alow-power-consumption light-emitting device which can be driven at lowvoltage can be achieved.

By making emission colors of the EL layers different, light of a desiredcolor can be obtained from the light-emitting element as a whole. Forexample, the emission colors of first and second EL layers arecomplementary in a light-emitting element having the two EL layers,whereby the light-emitting element can emit white light as a whole. Notethat the term “complementary” means color relationship in which anachromatic color is obtained when colors are mixed. In other words,emission of white light can be obtained by mixture of light emitted fromsubstances whose emission colors are complementary colors.

Further, the same applies to a light-emitting element having three ELlayers. For example, the light-emitting element as a whole can emitwhite light when the emission color of the first EL layer is red, theemission color of the second EL layer is green, and the emission colorof the third EL layer is blue.

As well as the structure described in this embodiment in which the ELlayers are stacked with the charge generation layer providedtherebetween, the light-emitting element may have a micro opticalresonator (microcavity) structure which utilizes a light resonant effectby adjusting a distance between the electrodes (the first electrode 301and the second electrode 304) to a desired value.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 4

In this embodiment, a light-emitting device including a light-emittingelement of one embodiment of the present invention is described.

Note that any of the light-emitting elements described in the otherembodiments can be used as the light-emitting element. Further, althougheither a passive matrix light-emitting device or an active matrixlight-emitting device may be used as the light-emitting device, anactive matrix light-emitting device is described in this embodiment withreference to FIGS. 5A and 5B.

Note that FIG. 5A is a top view illustrating a light-emitting device andFIG. 5B is a cross-sectional view taken along the chain line A-A′ inFIG. 5A. The active matrix light-emitting device of this embodimentincludes a pixel portion 502 provided over an element substrate 501, adriver circuit portion (a source line driver circuit) 503, and drivercircuit portions (gate line driver circuits) 504 a and 504 b. The pixelportion 502, the driver circuit portion 503, and the driver circuitportions 504 a and 504 b are sealed between the element substrate 501and the sealing substrate 506 with a sealant 505.

A lead wiring 507 is provided over the element substrate 501. The leadwiring 507 is provided for connecting an external input terminal throughwhich a signal (e.g., a video signal, a clock signal, a start signal,and a reset signal) or a potential from the outside is transmitted tothe driver circuit portion 503 and the driver circuit portions 504 a and504 b. Here is shown an example in which a flexible printed circuit(FPC) 508 is provided as the external input terminal. Although the FPCis illustrated alone, this FPC may be provided with a printed wiringboard (PWB). The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

Next, a cross-sectional structure is described with reference to FIG.5B. The driver circuit portion and the pixel portion are formed over theelement substrate 501; here are illustrated the driver circuit portion503 which is the source line driver circuit and the pixel portion 502.

The driver circuit portion 503 is an example where a CMOS circuit isformed, which is a combination of an n-channel FET 509 and a p-channelFET 510. Note that a circuit included in the driver circuit portion maybe formed using various CMOS circuits, PMOS circuits, or NMOS circuits.Although this embodiment shows a driver integrated type in which thedriver circuit is formed over the substrate, the driver circuit is notnecessarily formed over the substrate, and may be formed outside thesubstrate.

The pixel portion 502 is formed of a plurality of pixels each of whichincludes a switching FET 511, a current control FET 512, and a firstelectrode (anode) 513 which is electrically connected to a wiring (asource electrode or a drain electrode) of the current control FET 512.Note that an insulator 514 is formed to cover end portions of the firstelectrode (anode) 513. In this embodiment, the insulator 514 is formedusing a positive photosensitive acrylic resin.

The insulator 514 preferably has a curved surface with curvature at anupper end portion or a lower end portion thereof in order to obtainfavorable coverage by a film which is to be stacked over the insulator514. For example, in the case of using a positive photosensitive acrylicresin as a material of the insulator 514, the insulator 514 preferablyhas a curved surface with a curvature radius (0.2 μm to 3 μm) at theupper end portion. Note that the insulator 514 can be formed usingeither a negative photosensitive resin or a positive photosensitiveresin. The material of the insulator 514 is not limited to an organiccompound, and an inorganic compound such as silicon oxide or siliconoxynitride can also be used.

An EL layer 515 and a second electrode (cathode) 516 are stacked overthe first electrode (anode) 513, so that a light-emitting element 517 isformed. Note that the EL layer 515 includes at least the light-emittinglayer described in Embodiment 1. In the EL layer 515, a hole-injectionlayer, a hole-transport layer, an electron-transport layer, anelectron-injection layer, a charge-generation layer, and the like can beprovided as appropriate in addition to the light-emitting layer.

For the first electrode (anode) 513, the EL layer 515, and the secondelectrode (cathode) 516, the materials described in Embodiment 2 can beused. Although not illustrated, the second electrode (cathode) 516 iselectrically connected to the FPC 508 which is an external inputterminal.

Although the cross-sectional view of FIG. 5B illustrates only onelight-emitting element 517, a plurality of light-emitting elements isarranged in a matrix in the pixel portion 502. Light-emitting elementswhich provide three kinds of light emission (R, G, and B) areselectively formed in the pixel portion 502, whereby a light-emittingdevice capable of full color display can be fabricated. Alternatively, alight-emitting device which is capable of full color display may befabricated by a combination with color filters.

Further, the sealing substrate 506 is attached to the element substrate501 with the sealant 505, whereby the light-emitting element 517 isprovided in a space 518 surrounded by the element substrate 501, thesealing substrate 506, and the sealant 505. The space 518 may be filledwith an inert gas (such as nitrogen or argon), or the sealant 505.

An epoxy-based resin or a glass fit is preferably used for the sealant505. It is preferable that such a material allow permeation of moistureor oxygen as little as possible. As the sealing substrate 506, a glasssubstrate, a quartz substrate, or a plastic substrate formed offiberglass reinforced plastic (FRP), polyvinyl fluoride (PVF),polyester, acrylic, or the like can be used. In the case where glassfrit is used as the sealant, the element substrate 501 and the sealingsubstrate 506 are preferably glass substrates.

As described above, an active matrix light-emitting device can beobtained.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 5

In this embodiment, examples of a variety of electronic devices whichare completed using a light-emitting device are described with referenceto FIGS. 6A to 6D and FIGS. 7A to 7C. The light-emitting device isfabricated using the light-emitting element of one embodiment of thepresent invention.

Examples of the electronic devices to which the light-emitting device isapplied include television devices (also referred to as TV or televisionreceivers), monitors for computers and the like, digital cameras,digital video cameras, digital photo frames, mobile phones (alsoreferred to as cellular phones or mobile phone devices), portable gamemachines, portable information terminals, audio playback devices, andlarge-sized game machines such as pin-ball machines. Specific examplesof the electronic devices are illustrated in FIGS. 6A to 6D.

FIG. 6A illustrates an example of a television device. In a televisiondevice 7100, a display portion 7103 is incorporated in a housing 7101.Images can be displayed by the display portion 7103, and thelight-emitting device can be used for the display portion 7103. Inaddition, here, the housing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. With the receiver, a general television broadcastcan be received. Furthermore, when the television device 7100 isconnected to a communication network by wired or wireless connection viathe modem, one-way (from a transmitter to a receiver) or two-way(between a transmitter and a receiver, between receivers, or the like)data communication can be performed.

FIG. 6B illustrates a computer including a main body 7201, a housing7202, a display portion 7203, a keyboard 7204, an external connectingport 7205, a pointing device 7206, and the like. This computer ismanufactured by using the light-emitting device of one embodiment of thepresent invention for the display portion 7203.

FIG. 6C illustrates a portable game machine including two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.A display portion 7304 is incorporated in the housing 7301 and a displayportion 7305 is incorporated in the housing 7302. In addition, theportable game machine illustrated in FIG. 6C includes a speaker portion7306, a recording medium insertion portion 7307, an LED lamp 7308, aninput means (an operation key 7309, a connection terminal 7310, a sensor7311 (a sensor having a function of measuring force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, chemical substance,sound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, odor, or infraredrays), or a microphone 7312), and the like. It is needless to say thatthe structure of the portable game machine is not limited to the aboveas long as a light-emitting device is used for at least either thedisplay portion 7304 or the display portion 7305, or both, and mayinclude other accessories as appropriate. The portable game machineillustrated in FIG. 6C has a function of reading out a program or datastored in a storage medium to display it on the display portion, and afunction of sharing information with another portable game machine bywireless communication. The portable game machine illustrated in FIG. 6Ccan have a variety of functions without limitation to the above.

FIG. 6D illustrates an example of a mobile phone. A mobile phone 7400 isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone 7400is manufactured using a light-emitting device for the display portion7402.

When the display portion 7402 of the mobile phone 7400 illustrated inFIG. 6D is touched with a finger or the like, data can be input into themobile phone 7400. Further, operations such as making a call andcomposing an e-mail can be performed by touch on the display portion7402 with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be input. In this case, itis preferable to display a keyboard or number buttons on almost theentire screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside themobile phone 7400, display on the screen of the display portion 7402 canbe automatically changed by determining the orientation of the mobilephone 7400 (whether the mobile phone is placed horizontally orvertically).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on the kind of imagesdisplayed on the display portion 7402. For example, when a signal of animage displayed on the display portion is a signal of moving image data,the screen mode is switched to the display mode. When the signal is asignal of text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIGS. 7A and 7B illustrate a foldable tablet terminal. In FIG. 7A, thetablet terminal is opened. The tablet terminal includes a housing 9630,a display portion 9631 a, a display portion 9631 b, a display modeswitch 9034, a power switch 9035, a power saver switch 9036, a clasp9033, and an operation switch 9038. The tablet terminal is manufacturedusing the light-emitting device for one or both of the display portions9631 a and 9631 b.

Part of the display portion 9631 a can be a touch panel region 9632 a,and data can be input by touching operation keys 9637 that aredisplayed. Note that FIG. 7A shows, as an example, that half of the areaof the display portion 9631 a has only a display function and the otherhalf of the area has a touch panel function. However, the structure ofthe display portion 9631 a is not limited to this, and all the area ofthe display portion 9631 a may have a touch panel function. For example,all the area of the display portion 9631 a can display keyboard buttonsand serve as a touch panel while the display portion 9631 b can be usedas a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a finger, a stylus, or the liketouches the place where a button 9639 for switching to keyboard displayis displayed in the touch panel, keyboard buttons can be displayed onthe display portion 9631 b.

Furthermore, touch input can be performed concurrently on the touchpanel regions 9632 a and 9632 b.

The switch 9034 for switching display modes can switch displayorientation (e.g., between landscape mode and portrait mode) and selecta display mode (switch between monochrome display and color display),for example. With the switch 9036 for switching to power-saving mode,the luminance of display can be optimized in accordance with the amountof external light at the time when the tablet terminal is in use, whichis detected with an optical sensor incorporated in the tablet terminal.The tablet terminal may include another detection device such as asensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although FIG. 7A shows the example where the display area of the displayportion 9631 a is the same as that of the display portion 9631 b, oneembodiment of the present invention is not limited to this example. Theymay differ in size and/or image quality. For example, one of them may bea display panel that can display higher-definition images than theother.

FIG. 7B illustrates the tablet terminal which is closed. The tabletterminal includes the housing 9630, a solar battery 9633, acharge/discharge control circuit 9634, a battery 9635, and a DC to DCconverter 9636. As an example, FIG. 7B illustrates the charge/dischargecontrol circuit 9634 including the battery 9635 and the DC to DCconverter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when the tablet terminal is not in use. Thus, the displayportions 9631 a and 9631 b can be protected, thereby providing a tabletterminal with high endurance and high reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can also have afunction of displaying various kinds of data, such as a calendar, adate, or the time, on the display portion as a still image, a movingimage, and a text image, a function of displaying, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar battery 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch panel, a display portion,an image signal processor, and the like. Note that a structure in whichthe solar battery 9633 is provided is preferable because the battery9635 which supplies electric power to the display portion 9631 a and/orthe display portion 9631 b can be charged. When a lithium ion battery isused as the battery 9635, there is an advantage of downsizing or thelike.

The structure and operation of the charge/discharge control circuit 9634illustrated in FIG. 7B are described with reference to a block diagramin FIG. 7C. FIG. 7C illustrates the solar battery 9633, the battery9635, the DC to DC converter 9636, a converter 9638, switches SW1 toSW3, and the display portion 9631. The battery 9635, the DC to DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to those in the charge/discharge control circuit 9634illustrated in FIG. 7B.

An example of the operation performed when power is generated by thesolar battery 9633 using external light is described. The voltage ofpower generated by the solar battery 9633 is raised or lowered by the DCto DC converter 9636 so as to be a voltage for charging the battery9635. Then, when power from the solar battery 9633 is used for theoperation of the display portion 9631, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 9638 soas to be a voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Here, the solar battery 9633 is shown as an example of a powergeneration means; however, there is no particular limitation on a way ofcharging the battery 9635, and the battery 9635 may be charged withanother power generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 9635 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

It is needless to say that an embodiment of the present invention is notlimited to the electronic device illustrated in FIGS. 7A to 7C as longas the display portion described in the above embodiment is included.

As described above, the electronic devices can be obtained byapplication of the light-emitting device of one embodiment of thepresent invention. The light-emitting device has an extremely wideapplication range, and can be applied to electronic devices in a varietyof fields.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 6

In this embodiment, examples of lighting devices which are completedusing a light-emitting device are described with reference to FIG. 8.The light-emitting device is fabricated using a light-emitting elementof one embodiment of the present invention.

FIG. 8 illustrates an example in which the light-emitting device is usedas an indoor lighting device 8001. Since the light-emitting device canhave a larger area, it can be used for a lighting device having a largearea. In addition, a lighting device 8002 in which a light-emittingregion has a curved surface can also be obtained with the use of ahousing with a curved surface. A light-emitting element included in thelight-emitting device described in this embodiment is in a thin filmform, which allows the housing to be designed more freely. Therefore,the lighting device can be elaborately designed in a variety of ways.Further, a wall of the room may be provided with a large-sized lightingdevice 8003.

Moreover, when the light-emitting device is used at a surface of atable, a lighting device 8004 which has a function as a table can beobtained. When the light-emitting device is used as part of otherfurniture, a lighting device which has a function as the furniture canbe obtained.

As described above, a variety of lighting devices to, which thelight-emitting device is applied can be obtained. Note that suchlighting devices are also embodiments of the present invention.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments, as appropriate.

Embodiment 7

In this embodiment, a light-emitting device manufactured using thelight-emitting element of one embodiment of the present invention isdescribed with reference to FIGS. 18A and 18B.

In FIG. 18A, a plan view of a light-emitting device described in thisembodiment and a cross-sectional view taken along the dashed-dotted lineE-F in the plan view are illustrated.

The light-emitting device illustrated in FIG. 18A includes alight-emitting portion 2002 including a light-emitting element over afirst substrate 2001. The light-emitting device has a structure in whicha first sealant 2005 a is provided so as to surround the light-emittingportion 2002 and a second sealant 2005 b is provided so as to surroundthe first sealant 2005 a (i.e., a double sealing structure).

Thus, the light-emitting portion 2002 is positioned in a spacesurrounded by the first substrate 2001, the second substrate 2006, andthe first sealant 2005 a.

Note that in this specification, the first sealant 2005 a and the secondsealant 2005 b are not necessarily in contact with the first substrate2001 and the second substrate 2006. For example, the first sealant 2005a may be in contact with an insulating film or a conductive film formedover the first substrate 2001.

In the above structure, the first sealant 2005 a is a resin layercontaining a desiccant and the second sealant 2005 b is a glass layer,whereby an effect of suppressing entry of impurities such as moistureand oxygen from the outside (hereinafter, referred to as a sealingproperty) can be increased.

The first sealant 2005 a is the resin layer as described above, wherebythe glass layer that is the second sealant 2005 b can be prevented fromhaving breaking or cracking (hereinafter, collectively referred to as acrack). Further, in the case where the sealing property of the secondsealant 2005 b is not sufficient, even when impurities such as moistureand oxygen enter a first space 2013, entry of the impurities such asmoisture and oxygen into a second space 2011 can be suppressed owing toa high sealing property of the first sealant 2005 a. Thus, deteriorationof an organic compound, a metal material, and the like contained in thelight-emitting element because of entry of impurities such as moistureand oxygen into the light-emitting portion 2002 can be suppressed.

In addition, the structure illustrated in FIG. 18B can be employed inwhich the first sealant 2005 a is a glass layer and the second sealant2005 b is a resin layer containing a desiccant.

In each of the light-emitting devices described in this embodiment,distortion due to external force or the like increases toward the outerportion of the light-emitting device. In view of the above, the firstsealant 2005 a which has relatively small distortion due to externalforce or the like is a glass layer and the second sealant 2005 b is aresin layer which has excellent impact resistance and excellent heatresistance and is not easily broken by deformation due to external forceor the like, whereby entry of moisture and oxygen into the first space2013 can be suppressed.

In addition to the above structure, a material serving as a desiccantmay be contained in each of the first space 2013 and the second space2011.

In the case where the first sealant 2005 a or the second sealant 2005 bis a glass layer, for example, a glass frit or a glass ribbon can beused. Note that at least a glass material is contained in a glass fritor a glass ribbon.

The glass frit contains a glass material as a frit material. The glassfrit may contain, for example, magnesium oxide, calcium oxide, strontiumoxide, barium oxide, cesium oxide, sodium oxide, potassium oxide, boronoxide, vanadium oxide, zinc oxide, tellurium oxide, aluminum oxide,silicon dioxide, lead oxide, tin oxide, phosphorus oxide, rutheniumoxide, rhodium oxide, iron oxide, copper oxide, manganese dioxide,molybdenum oxide, niobium oxide, titanium oxide, tungsten oxide, bismuthoxide, zirconium oxide, lithium oxide, antimony oxide, lead borateglass, tin phosphate glass, vanadate glass, or borosilicate glass. Theglass frit preferably contains at least one kind of transition metal toabsorb infrared light.

In the case where a glass layer is formed using any of the above glassfrits, for example, a frit paste is applied to a substrate and issubjected to heat treatment, laser light irradiation, or the like. Thefrit paste contains the frit material and a resin (also referred to as abinder) diluted by an organic solvent. A known material and structurecan be used for the fit paste. An absorber which absorbs light having awavelength of laser light may be added to the frit material. Forexample, an Nd:YAG laser or a semiconductor laser is preferably used asthe laser. The shape of laser light may be circular or quadrangular.

Note that the thermal expansion coefficient of the glass layer to beformed is preferably close to that of the substrate. The closer thethermal expansion coefficients are, the more generation of a crack inthe glass layer or the substrate due to thermal stress can besuppressed.

Although any of known materials, for example, photocurable resins suchas an ultraviolet curable resin and thermosetting resins can be used inthe case where the first sealant 2005 a or the second sealant 2005 b isa resin layer, it is particularly preferable to use a material whichdoes not transmit moisture or oxygen. In particular, a photocurableresin is preferably used. The light-emitting element contains a materialhaving low heat resistance in some cases. A photocurable resin, which iscured by light irradiation, is preferably used, in which case change infilm quality and deterioration of an organic compound itself caused byheating of the light-emitting element can be suppressed. Furthermore,any of the organic compounds that can be used for the light-emittingelement of one embodiment of the present invention may be used.

As the desiccant contained in the resin layer, the first space 2013, orthe second space 2011, a known material can be used. As the desiccant, asubstance which adsorbs moisture and the like by chemical adsorption ora substance which adsorbs moisture and the like by physical adsorptioncan be used. Examples thereof are alkali metal oxides, alkaline earthmetal oxides (e.g., calcium oxide and barium oxide), sulfates, metalhalides, perchlorates, zeolite, and silica gel.

One or both of the first space 2013 and the second space 2011 may befilled with, for example, an inert gas such as a rare gas or a nitrogengas or may be filled with an organic resin. Note that these spaces areeach in an atmospheric pressure state or a reduced pressure state.

As described above, the light-emitting device described in thisembodiment has a double sealing structure, in which one of the firstsealant 2005 a and the second sealant 2005 b is the glass layer havingexcellent productivity and an excellent sealing property, and the otheris the resin layer which is hardly broken caused by external force orthe like, and can contain the desiccant inside, so that a sealingproperty of suppressing entry of impurities such as moisture and oxygenfrom the outside can be improved.

Thus, the use of the structure described in this embodiment can providea light-emitting device in which deterioration of a light-emittingelement due to impurities such as moisture and oxygen is suppressed.

Note that the structure described in this embodiment can be used incombination with any of the structures described in the otherembodiments and examples as appropriate.

Embodiment 8

In this embodiment, a light-emitting device in which the light-emittingelement of one embodiment of the present invention is used is describedwith reference to FIGS. 19A and 19B.

FIGS. 19A and 19B are each an example of a cross-sectional view of alight-emitting device including a plurality of light-emitting elements.A light-emitting device 3000 illustrated in FIG. 19A includeslight-emitting elements 3020 a, 3020 b, and 3020 c.

The light-emitting device 3000 includes island-shaped lower electrodes3003 a, 3003 b, and 3003 c over a substrate 3001. The lower electrodes3003 a, 3003 b, and 3003 c can function as anodes of the respectivelight-emitting elements. Reflective electrodes may be provided under thelower electrodes 3003 a, 3003 b, and 3003 c. Transparent conductivelayers 3005 a, 3005 b, and 3005 c may be provided over the lowerelectrodes 3003 a, 3003 b, and 3003 c, respectively. The transparentconductive layers 3005 a, 3005 b, and 3005 c preferably have differentthicknesses depending on emission colors of the elements.

The light-emitting device 3000 includes partitions 3007 a, 3007 b, 3007c, and 3007 d. Specifically, the partition 3007 a covers one edgeportion of the lower electrode 3003 a and one edge portion of thetransparent conductive layer 3005 a; the partition 3007 b covers theother edge portion of the lower electrode 3003 a and the other edgeportion of the transparent conductive layer 3005 a and also covers oneedge portion of the lower electrode 3003 b and one edge portion of thetransparent conductive layer 3005 b; the partition 3007 c covers theother edge portion of the lower electrode 3003 b and the other edgeportion of the transparent conductive layer 3005 b and also covers oneedge portion of the lower electrode 3003 c and one edge portion of thetransparent conductive layer 3005 c; the partition 3007 d covers theother edge portion of the lower electrode 3003 c and the other edgeportion of the transparent conductive layer 3005 c.

The light-emitting device 3000 includes a hole-injection layer 3009 overthe lower electrodes 3003 a, 3003 b, and 3003 c and the partitions 3007a, 3007 b, 3007 c, and 3007 d.

The light-emitting device 3000 includes a hole-transport layer 3011 overthe hole-injection layer 3009. The light-emitting device 3000 alsoincludes light-emitting layers 3013 a, 3013 b, and 3013 c over thehole-transport layer 3011. The light-emitting device 3000 also includesan electron-transport layer 3015 over the light-emitting layers 3013 a,3013 b, and 3013 c.

Further, the light-emitting device 3000 includes an electron-injectionlayer 3017 over the electron-transport layer 3015. The light-emittingdevice 3000 also includes an upper electrode 3019 over theelectron-injection layer 3017. The upper electrode 3019 can function ascathodes of the light-emitting elements.

Note that although an example in which the lower electrodes 3003 a, 3003b, and 3003 c function as the anodes of the light-emitting elements andthe upper electrode 3019 functions as the cathodes of the light-emittingelements is described with reference to FIG. 19A, the stacking order ofthe anode and the cathode may be switched. In this case, the stackingorder of the electron-injection layer, the electron-transport layer, thehole-transport layer, and the hole-injection layer may be changed asappropriate.

The light-emitting element of one embodiment of the present inventioncan be applied to the light-emitting layers 3013 a, 3013 b, and 3013 c.The light-emitting element can have low driving voltage, high currentefficiency, or a long lifetime; thus, the light-emitting device 3000 canhave low power consumption or a long lifetime.

A light-emitting device 3100 illustrated in FIG. 19B includeslight-emitting elements 3120 a, 3120 b, and 3120 c. The light-emittingelements 3120 a, 3120 b, and 3120 c are tandem light-emitting elementsin which a plurality of light-emitting layers is provided between lowerelectrodes 3103 a, 3103 b, and 3103 c and an upper electrode 3119.

The light-emitting device 3100 includes the island-shaped lowerelectrodes 3103 a, 3103 b, and 3103 c over a substrate 3101. The lowerelectrodes 3103 a, 3103 b, and 3103 c function as anodes of thelight-emitting elements. Note that reflective electrodes may be providedunder the lower electrodes 3103 a, 3103 b, and 3103 c. Transparentconductive layers 3105 a and 3105 b may be provided over the lowerelectrodes 3103 a and 3103 b. The transparent conductive layers 3105 aand 3105 b preferably have different thicknesses depending on emissioncolors of the elements. Although not illustrated, a transparentconductive layer may also be provided over the lower electrode 3103 c.

The light-emitting device 3100 includes partitions 3107 a, 3107 b, 3107c, and 3107 d. Specifically, the partition 3107 a covers one edgeportion of the lower electrode 3103 a and one edge portion of thetransparent conductive layer 3105 a; the partition 3107 b covers theother edge portion of the lower electrode 3103 a and the other edgeportion of the transparent conductive layer 3105 a and also covers oneedge portion of the lower electrode 3103 b and one edge portion of thetransparent conductive layer 3105 b; the partition 3107 c covers theother edge portion of the lower electrode 3103 b and the other edgeportion of the transparent conductive layer 3105 b and also covers oneedge portion of the lower electrode 3103 c and one edge portion of thetransparent conductive layer 3105 c; the partition 3107 d covers theother edge portion of the lower electrode 3103 c and the other edgeportion of the transparent conductive layer 3105 c.

The light-emitting device 3100 includes a hole-injection andhole-transport layer 3110 over the lower electrodes 3103 a, 3103 b, and3103 c and the partitions 3107 a, 3107 b, 3107 c, and 3107 d.

The light-emitting device 3100 includes a first light-emitting layer3112 over the hole-injection and hole-transport layer 3110. Thelight-emitting device 3100 also includes a second light-emitting layer3116 over the first light-emitting layer 3112 with a charge generationlayer 3114 therebetween.

Further, the light-emitting device 3100 includes an electron-transportand electron-injection layer 3118 over the second light-emitting layer3116. In addition, the light-emitting device 3100 includes the upperelectrode 3119 over the electron-transport and electron-injection layer3118. The upper electrode 3119 can function as cathodes of thelight-emitting elements.

Note that although an example in which the lower electrodes 3103 a, 3103b, and 3103 c function as the anodes of the light-emitting elements andthe upper electrode 3119 functions as the cathodes of the light-emittingelements is described with reference to FIG. 19B, the stacking order ofthe anode and the cathode may be switched. In this case, the stackingorder of the electron-injection layer, the electron-transport layer, thehole-transport layer, and the hole-injection layer may be changed asappropriate.

The light-emitting element of one embodiment of the present inventioncan be applied to the first light-emitting layer 3112 and the secondlight-emitting layer 3116. The light-emitting element can have lowdriving voltage, high current efficiency, or a long lifetime; thus, thelight-emitting device 3100 can have low power consumption or a longlifetime.

Note that the structure described in this embodiment can be combinedwith any of the structures described in the other embodiments and theexamples as appropriate.

Embodiment 9

In this embodiment, a lighting device manufactured using thelight-emitting element of one embodiment of the present invention isdescribed with reference to FIGS. 20A to 20E.

FIGS. 20A to 20E are a plan view and cross-sectional views of lightingdevices. FIGS. 20A to 20C are bottom-emission lighting devices in whichlight is extracted from the substrate side. FIG. 20B is across-sectional view taken along the dashed-dotted line G-H in FIG. 20A.

A lighting device 4000 illustrated in FIGS. 20A and 20B includes alight-emitting element 4007 over a substrate 4005. In addition, thelighting device 4000 includes a substrate 4003 with unevenness on theoutside of the substrate 4005. The light-emitting element 4007 includesa lower electrode 4013, an EL layer 4014, and an upper electrode 4015.

The lower electrode 4013 is electrically connected to an electrode 4009,and the upper electrode 4015 is electrically connected to an electrode4011. An auxiliary wiring 4017 electrically connected to the lowerelectrode 4013 may be provided.

The substrate 4005 and a sealing substrate 4019 are bonded to each otherby a sealant 4021. A desiccant 4023 is preferably provided between thesealing substrate 4019 and the light-emitting element 4007.

The substrate 4003 has the unevenness as illustrated in FIG. 20A,whereby the extraction efficiency of light emitted from thelight-emitting element 4007 can be increased. Instead of the substrate4003, a diffusion plate 4027 may be provided on the outside of thesubstrate 4025 as in a lighting device 4001 illustrated in FIG. 20C.

FIGS. 20D and 20E illustrate top-emission lighting devices in whichlight is extracted from the side opposite to the substrate.

A lighting device 4100 illustrated in FIG. 20D includes a light-emittingelement 4107 over a substrate 4125. The light-emitting element 4107includes a lower electrode 4113, an EL layer 4114, and an upperelectrode 4115.

The lower electrode 4113 is electrically connected to an electrode 4109,and the upper electrode 4115 is electrically connected to an electrode4111. An auxiliary wiring 4117 electrically connected to the upperelectrode 4115 may be provided. An insulating layer 4131 may be providedunder the auxiliary wiring 4117.

The substrate 4125 and a sealing substrate 4103 with unevenness arebonded to each other by a sealant 4121. A planarization film 4105 and abarrier film 4129 may be provided between the sealing substrate 4103 andthe light-emitting element 4107.

The sealing substrate 4103 has the unevenness as illustrated in FIG.20D, the extraction efficiency of light emitted from the light-emittingelement 4107 can be increased. Instead of the sealing substrate 4103, adiffusion plate 4127 may be provided over the light-emitting element4107 as in a lighting device 4101 illustrated in FIG. 20E.

The light-emitting element of one embodiment of the present inventioncan be applied to light-emitting layers included in the EL layer 4014and the EL layer 4114. The light-emitting element can have low drivingvoltage, high current efficiency, or a long lifetime; thus, the lightingdevices 4000, 4001, 4100, and 4101 can have low power consumption or along lifetime.

Note that the structure described in this embodiment can be combinedwith any of the structures described in the other embodiments and theexamples as appropriate.

Embodiment 10

In this embodiment, a touch sensor and a module each of which can becombined with the light-emitting device of one embodiment of the presentinvention are described with reference to FIGS. 21A and 21B, FIG. 22,FIG. 23, and FIG. 24.

FIG. 21A is an exploded perspective view illustrating a structuralexample of a touch sensor 4500. FIG. 21B is a plan view illustrating astructural example of the touch sensor 4500.

The touch sensor 4500 illustrated in FIGS. 21A and 21B includes, over asubstrate 4910, a plurality of conductive layers 4510 arranged in theX-axis direction and a plurality of conductive layers 4520 arranged inthe Y-axis direction which intersect with the X-axis direction. In FIGS.21A and 21B illustrating the touch sensor 4500, a plane over which theplurality of conductive layers 4510 are formed and a plane over whichthe plurality of conductive layers 4520 are formed are separatelyillustrated.

FIG. 22 is an equivalent circuit diagram illustrating the portion wherethe conductive layer 4510 and the conductive layer 4520 of the touchsensor 4500 illustrated in FIGS. 21A and 21B intersect with each other.A capacitor 4540 is formed in the portion where the conductive layer4510 and the conductive layer 4520 intersect with each other as in FIG.22.

The conductive layer 4510 and the conductive layer 4520 each have astructure in which a plurality of quadrangular conductive films isconnected to one another. The plurality of conductive layers 4510 andthe plurality of conductive layers 4520 are provided so that thequadrangular conductive films of the conductive layer 4510 and thequadrangular conductive films of the conductive layer 4520 do notoverlap with each other. In the portion where the conductive layer 4510intersects with the conductive layer 4520, an insulating film isprovided between the conductive layer 4510 and the conductive layer 4520so that the conductive layer 4510 and the conductive layer 4520 are notin contact with each other.

FIG. 23 is a cross-sectional view illustrating an example of aconnection between the conductive layers 4510 a, 4510 b, and 4510 c andthe conductive layer 4520 in the touch sensor 4500 illustrated in FIGS.21A and 21B, and is an example of a cross-sectional view illustrating aportion where the conductive layer 4510 (conductive layers 4510 a, 4510b, and 4510 c) intersect with the conductive layer 4520.

As illustrated in FIG. 23, the conductive layer 4510 includes theconductive layer 4510 a and the conductive layer 4510 b in the firstlayer and the conductive layer 4510 c in the second layer over aninsulating layer 4810. The conductive layer 4510 a and the conductivelayer 4510 b are connected to each other by the conductive layer 4510 c.The conductive layer 4520 is formed using the conductive layer in thefirst layer. The insulating layer 4820 is formed so as to cover theconductive layers 4510 and 4520 and part of a conductive layer 4710. Asthe insulating layers 4810 and 4820, for example, a silicon oxynitridefilm may be formed. Note that a base film formed of an insulating filmmay be formed between a substrate 4910 and the conductive layers 4710,4510 a, 4510 b, and 4520. As the base film, for example, a siliconoxynitride film can be formed.

The conductive layers 4510 a, 4510 b, and 4510 c and the conductivelayer 4520 are formed using a conductive material having a property oftransmitting visible light. Examples of the conductive material having aproperty of transmitting visible light include indium tin oxidecontaining silicon oxide, indium tin oxide, zinc oxide, indium zincoxide, and zinc oxide to which gallium is added.

The conductive layer 4510 a is connected to the conductive layer 4710. Aterminal for connection to an FPC is formed using the conductive layer4710. The conductive layer 4520 is connected to the conductive layer4710 like the conductive layer 4510 a. The conductive layer 4710 can beformed of, for example, a tungsten film.

The insulating layer 4820 is formed so as to cover the conductive layers4510 and 4520 and the conductive layer 4710. An opening is formed in theinsulating layers 4810 and 4820 over the conductive layer 4710 so thatthe conductive layer 4710 is electrically connected to an FPC. Asubstrate 4920 is attached to and over the insulating layer 4820 usingan adhesive, an adhesive film, or the like. The substrate 4910 side isbonded to a color filter substrate of a display panel with an adhesiveor an adhesive film, so that a touch panel is completed.

Next, a module for which the light-emitting device of one embodiment ofthe present invention can be used is described with reference to FIG.24.

In a module 5000 illustrated in FIG. 24, a touch panel 5004 connected toan FPC 5003, a display panel 5006 connected to an FPC 5005, a backlightunit 5007, a frame 5009, a printed board 5010, and a battery 5011 areprovided between an upper cover 5001 and a lower cover 5002.

The shapes and sizes of the upper cover 5001 and the lower cover 5002can be changed as appropriate in accordance with the sizes of the touchpanel 5004 and the display panel 5006.

The touch panel 5004 can be a resistive touch panel or a capacitivetouch panel and can be formed to overlap with the display panel 5006. Itis also possible to provide a touch panel function for a countersubstrate (sealing substrate) of the display panel 5006. A photosensormay be provided in each pixel of the display panel 5006 so that anoptical touch panel is obtained.

The backlight unit 5007 includes light sources 5008. Note that althougha structure in which the light sources 5008 are provided over thebacklight unit 5007 is illustrated in FIG. 24, one embodiment of thepresent invention is not limited to this structure. For example, astructure in which a light source 5008 is provided at an end portion ofthe backlight unit 5007 and a light diffusion plate is further providedmay be employed.

The frame 5009 has a function of protecting the display panel 5006 andfunctions as an electromagnetic shield for blocking electromagneticwaves generated by the operation of the printed board 5010. The frame5009 may function as a radiator plate.

The printed board 5010 has a power supply circuit and a signalprocessing circuit for outputting a video signal and a clock signal. Asa power source for supplying electric power to the power supply circuit,an external commercial power source or a power source using a battery5011 separately provided may be used. The battery 5011 can be omittedwhen a commercial power source is used.

The module 5000 can be additionally provided with a member such as apolarizing plate, a retardation plate, or a prism sheet.

Note that the structure described in this embodiment can be combinedwith any of the structures described in the other embodiments and theexamples as appropriate.

Embodiment 11

In this embodiment, a structure of a light-emitting element of oneembodiment of the present invention is described with reference to FIGS.25A and 25B.

A light-emitting element 6002 illustrated in FIG. 25A is formed over asubstrate 6001. The light-emitting element 6002 includes a firstelectrode 6003, an EL layer 6004, and a second electrode 6005. In alight-emitting device illustrated in FIG. 25A, a buffer layer 6006 isformed over the second electrode 6005, and a third electrode 6007 isformed over the buffer layer 6006. The buffer layer 6006 can prevent adecrease in light-extraction efficiency due to surface plasmon generatedon a surface of the second electrode 6005.

Note that the second electrode 6005 and the third electrode 6007 areelectrically connected to each other in a contact portion 6008. Theposition of the contact portion 6008 is not limited to the position inthe drawing, and may be formed in a light-emitting region.

The first electrode 6003 may be an anode and the second electrode 6005may be a cathode, or alternatively, the first electrode 6003 may be acathode and the second electrode 6005 may be an anode. At least one ofthe electrodes has a light-transmitting property, and both of theelectrodes may be formed with light-transmitting materials. In the casewhere the first electrode 6003 has a function of transmitting light fromthe EL layer 6004, a transparent conductive film such as ITO can be usedfor the first electrode 6003. In the case where the first electrode 6003blocks light from the EL layer 6004, a conductive film formed bystacking a plurality of layers (e.g., ITO and silver) can be used forthe first electrode 6003.

In a structure in which light from the EL layer 6004 is extracted on thefirst electrode 6003 side, the thickness of the second electrode 6005 ispreferably smaller than the thickness of the third electrode 6007. In astructure in which the light is extracted on the opposite side, thethickness of the second electrode 6005 is preferably larger than thethickness of the third electrode 6007. However, the thickness is notlimited thereto.

For the buffer layer 6006, an organic resin film (e.g., Alq(abbreviation)), an inorganic insulating material (e.g., a siliconnitride film), or the like can be used.

The light-extraction efficiency may be improved by employing a structureillustrated in FIG. 25B as a structure including the light-emittingelement of one embodiment of the present invention.

In the structure illustrated in FIG. 25B, a light scattering layer 6100including a light scatterer 6101 and an air layer 6102 is formed incontact with the substrate 6001; a high refractive index layer 6103formed with an organic resin is formed in contact with the lightscattering layer 6100; and an element layer 6104 including alight-emitting element and the like is formed in contact with the highrefractive index layer 6103.

For the light scatterer 6101, particles of ceramic or the like can beused. For the high refractive index layer 6103, a high refractive index(e.g., refractive index of 1.7 to 1.8) material such as polyethylenenaphthalate (PEN) can be used.

The element layer 6104 includes the light-emitting element described inthis specification and the like.

EXAMPLE 1

In this example, a light-emitting element 1 and a comparativelight-emitting element 2 which are embodiments of the present inventionare described with reference to FIG. 9. Chemical formulae of materialsused in this example are shown below.

<<Fabrication of Light-Emitting Element 1 and Comparative Light-EmittingElement 2>>

First, a film of indium oxide-tin oxide containing silicon oxide (ITSO)was formed over a glass substrate 1100 by a sputtering method, so that afirst electrode 1101 functioning as an anode was formed. The thicknesswas 110 nm and the electrode area was 2 mm×2 mm.

Next, as pretreatment for forming the light-emitting element over thesubstrate 1100, the surface of the substrate was washed with water,baked at 200° C. for 1 hour, and subjected to UV ozone treatment for 370seconds.

After that, the substrate 1100 was transferred into a vacuum evaporationapparatus in which the pressure had been reduced to approximately 10⁻⁴Pa, and subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate 1100 was cooled down for about 30 minutes.

Then, the substrate 1100 over which the first electrode 1101 was formedwas fixed to a substrate holder provided in the vacuum evaporationapparatus so that the surface provided with the first electrode 1101faced downward. In this example, a case is described in which ahole-injection layer 1111, a hole-transport layer 1112, a light-emittinglayer 1113, an electron-transport layer 1114, and an electron-injectionlayer 1115 which are included in an EL layer 1102 are sequentiallyformed by a vacuum evaporation method.

After reducing the pressure in the vacuum evaporation apparatus to 10⁻⁴Pa, 1,3,5-tri(dibenzothiophen-4-yl)benzene (abbreviation: DBT3P-II) andmolybdenum(VI) oxide were co-evaporated with a mass ratio of DBT3P-II(abbreviation) to molybdenum oxide being 1:0.5, whereby thehole-injection layer 1111 was formed over the first electrode 1101. Thethickness was 20 nm. Note that a co-evaporation method is an evaporationmethod in which a plurality of different substances is concurrentlyvaporized from respective different evaporation sources.

Then, 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine (abbreviation:BPAFLP) was evaporated to a thickness of 20 nm, so that thehole-transport layer 1112 was formed.

Next, the light-emitting layer 1113 was formed over the hole-transportlayer 1112. For the light-emitting element 1,2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),4-(1-naphthyl)-4′-phenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBiNB), and(acetylacetonato)bis(6-tert-butyl-4-phenylpyrimidinato)iridium(III)(abbreviation: [Ir(tBuppm)₂(acac)]) were co-evaporated to a thickness of20 nm with a mass ratio of 2mDBTBPDBq-II to PCBBiNB and[Ir(tBuppm)₂(acac)] being 0.7:0.3:0.06, and then further co-evaporatedto a thickness of 20 nm with a mass ratio of 2mDBTBPDBq-II to PCBBiNBand [Ir(tBuppm)₂(acac)] being 0.8:0.2:0.06; thus, the light-emittinglayer 1113 was formed.

For the comparative light-emitting element 2, 2mDBTBPDBq-II,4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBA1BP), and [Ir(tBuppm)₂(acac)] were co-evaporated to a thickness of20 nm with a mass ratio of 2mDBTBPDBq-II to PCBA1BP and[Ir(tBuppm)₂(acac)] being 0.7:0.3:0.06, and then further co-evaporatedto a thickness of 20 nm with a mass ratio of 2mDBTBPDBq-II to PCBA1BPand [Ir(tBuppm)₂(acac)] being 0.8:0.2:0.06; thus, the light-emittinglayer 1113 was formed.

Then, 2mDBTBPDBq-II was evaporated to a thickness of 10 nm over thelight-emitting layer 1113 and bathophenanthroline (abbreviation: Bphen)was evaporated to a thickness of 15 nm, whereby the electron-transportlayer 1114 having a stacked structure was formed. Furthermore, lithiumfluoride was evaporated to a thickness of 1 nm over theelectron-transport layer 1114, whereby the electron-injection layer 1115was formed.

Finally, aluminum was evaporated to a thickness of 200 nm over theelectron-injection layer 1115 to form a second electrode 1103 serving asa cathode; thus, the light-emitting element 1 and the comparativelight-emitting element 2 were obtained. Note that, in the aboveevaporation process, evaporation was all performed by a resistanceheating method.

In the above-described manner, the light-emitting element 1 and thecomparative light-emitting element 2 were obtained. Table 1 showselement structures of the light-emitting element 1 and the comparativelight-emitting element 2.

TABLE 1 Hole- Hole- Light- Electron- First injection transport emittingElectron-transport injection Second electrode layer layer layer layerlayer electrode Light- ITSO DBT3P-II:MoOx BPAFLP * ** 2mDBTBPDBq-IIBphen LiF Al emitting (110 nm) (1:0.5 20 nm) (20 nm) (10 nm) (15 nm) (1nm) (200 nm) element 1 Comparative *** **** light- emitting element 2 *2mDBTBPDBq-II:PCBBiNB:[Ir(tBuppm)₂(acac)] (0.7:0.3:0.06 20 nm) **2mDBTBPDBq-II:PCBBiNB:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm) ***2mDBTBPDBq-II:PCBA1BP:[Ir(tBuppm)₂(acac)] (0.7:0.3:0.06 20 nm) ****2mDBTBPDBq-II:PCBA1BP:[Ir(tBuppm)₂(acac)] (0.8:0.2:0.06 20 nm)

The fabricated light-emitting element 1 and comparative light-emittingelement 2 were sealed in a glove box containing a nitrogen atmosphere soas not to be exposed to the air (specifically, a sealant was appliedonto outer edges of the elements and heat treatment was performed at 80°C. for 1 hour at the time of sealing).

<<Operation Characteristics of Light-Emitting Element 1 and ComparativeLight-Emitting Element 2>>

Operation characteristics of the fabricated light-emitting element 1 andcomparative light-emitting element 2 were measured. Note that themeasurement was carried out at room temperature (in an atmosphere keptat 25° C.).

FIG. 10 shows current density versus luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.In FIG. 10, the vertical axis represents luminance (cd/m²) and thehorizontal axis represents current density (mA/cm²). FIG. 11 showsvoltage versus luminance characteristics of the light-emitting element 1and the comparative light-emitting element 2. In FIG. 11, the verticalaxis represents luminance (cd/m²) and the horizontal axis representsvoltage (V). FIG. 12 shows luminance versus current efficiencycharacteristics of the light-emitting element 1 and the comparativelight-emitting element 2. In FIG. 12, the vertical axis representscurrent efficiency (cd/A) and the horizontal axis represents luminance(cd/m²). FIG. 13 shows voltage versus current characteristics of thelight-emitting element 1 and the comparative light-emitting element 2.In FIG. 13, the vertical axis represents current (mA) and the horizontalaxis represents voltage (V).

The results of FIG. 10, FIG. 11, FIG. 12, and FIG. 13 reveal thefollowing: there is little difference in element characteristics betweenthe light-emitting element 1 of one embodiment of the present inventionand the comparative light-emitting element 2; the light-emitting element1 has favorable characteristics (see Table 3 given below) though in alight-emitting layer of the light-emitting element 1, PCBBiNB whose T1level is lower than that of [Ir(tBuppm)₂(acac)] is used as a hostmaterial and [Ir(tBuppm)₂(acac)] is used as a guest material while in alight-emitting layer of the comparative light-emitting element 2,PCBA1BP whose T1 level is higher than that of [Ir(tBuppm)₂(acac)] isused as a host material.

Table 2 shows initial values of main characteristics of thelight-emitting element 1 and the comparative light-emitting element 2 ata luminance of about 1000 cd/m².

[Table 2]

TABLE 2 External Current Current Power quantum Voltage Current densityChromaticity Luminance efficiency efficiency efficiency (V) (mA)(mA/cm²) (x,y) (cd/m²) (cd/A) (lm/W) (%) Light- 3.1 0.056 1.4 (0.41,0.58) 1000 72 73 20 emitting element 1 Comparative 3.2 0.052 1.3 (0.41,0.58) 920 71 70 20 light- emitting element 2

The above results in Table 2 also show that each of the light-emittingelement 1 and the comparative light-emitting element 2 fabricated inthis example has high quantum efficiency.

FIG. 14 shows an emission spectrum of the light-emitting element 1 whichwas obtained when a current of 0.1 mA flowed in the light-emittingelement 1. As shown in FIG. 14, the emission spectrum of thelight-emitting element 1 has a peak at around 546 nm, which indicatesthat the emission spectrum is derived from emission of[Ir(tBuppm)₂(acac)] contained in the light-emitting layer 1113.

Next, reliability tests of the light-emitting element 1 and thecomparative light-emitting element 2 were conducted. FIG. 15 showsresults of the reliability tests. In FIG. 15, the vertical axisrepresents normalized luminance (%) with an initial luminance of 100%,and the horizontal axis represents driving time (h) of the elements.Note that in the reliability tests, the light-emitting element 1 and thecomparative light-emitting element 2 were driven under the conditionsthat the initial luminance was set to 5000 cd/m² and the current densitywas constant. As a result, the luminance of the light-emitting element 1after 100-hour driving was about 92% of the initial luminance; thus, thelight-emitting element 1 kept higher luminance than that of thecomparative light-emitting element 2.

The above reliability tests show that the light-emitting element 1 ofone embodiment of the present invention has high reliability and a longlifetime.

FIG. 16 shows measurement results of the T1 levels of PCBBiNB used inthe light-emitting layer of the light-emitting element 1, PCBA1BP usedin the light-emitting layer of the comparative light-emitting element 2,and [ft(tBuppm)₂(acac)] used in the light-emitting layers of thelight-emitting element 1 and the comparative light-emitting element 2 inthis example.

Note that the T1 levels were obtained by measurement of emission ofphosphorescence from the materials. In the measurement, each materialwas irradiated with excitation light with a wavelength of 325 mn and themeasurement temperature was 10 K. Note that time-resolved measurementusing mechanical choppers was employed for PCBBiNB and PCBA1BP whilenormal phosphorescence measurement without conducting time-resolvedmeasurement was employed for [Ir(tBuppm)₂(acac)]. In measuring an energylevel, calculation from an absorption wavelength is more accurate thancalculation from an emission wavelength. However, here, absorption ofthe T1 level was extremely low and measuring it is difficult; thus, theT1 level was measured by measuring an emission wavelength. For thisreason, a few errors may be included in the measured values.

Table 3 shows the measurement results.

TABLE 3 PCBBiNB PCBA1BP [Ir(tBuppm)₂(acac)] T1 level 2.23 eV 2.46 eV2.25 eV (557 nm) (504 nm) (551 nm)

Thus, it was confirmed that in the light-emitting element 1 usingPCBBiNB as a host material, the T1 level of the host material is lowerthan the T1 level of a guest material, and in the comparativelight-emitting element 2 using PCBA1BP as a host material, the T1 levelof the host material is higher than the T1 level of a guest material.According to the above results, it is found that the light-emittingelement 1 using PCBBiNB, which is chemically stable and has low T1level, has element characteristics as good as those of the comparativelight-emitting element 2.

EXAMPLE 2

In this example, a mass ratio of samples in which an organic film(thickness: 50 nm) was provided between quartz substrates werefabricated. For the organic film, PCBBiNB (abbreviation), PCBA1BP(abbreviation), and [Ir(tBuppm)₂(acac)] (abbreviation), which werecontained in at least one of the light-emitting layers of thelight-emitting element 1 and the comparative light-emitting element 2 inExample 1, were used, and the composition of these materials was made tobe different among the samples. The lifetime (τ₁, τ₂) [μsec] of eachsample was measured.

The mass ratio in the organic film was such that 2mDBTBPDBq-II: PCBBiNB(or PCBA1BP): [Ir(tBuppm)₂(acac)]=1−X:X:0.06. Table 4 shows thestructures of the samples.

TABLE 4 Sample X τ1 [μsec] τ2 [μsec] 1 0 (PCBBiNB: 0%) 1.15 — 2 0.2(PCBBiNB: 20%) 1.01 1.81 3 0.5 (PCBBiNB: 50%) 0.96 2.21 4 1 (PCBBiNB:100%) 1.00 3.42 5 1 (PCBA1BP: 50%) 1.20 —

For the measurement, each sample was irradiated with excited lighthaving a wavelength of 337 nm (500 ps), the hole size was set to 100 μm,and the measurement time was set in a range of 0 μsec to 20 μsec. FIG.17 shows the measurement results.

The results in FIG. 17 indicate that in the case where PCBA1BP whose T1level is higher than that of [Ir(tBuppm)₂(acac)] serving as a guestmaterial is used as a host material, the lifetime is represented by aone-component decay curve. On the other hand, in the case where PCBBiNBwhose T1 level is lower than that of [Ir(tBuppm)₂(acac)] serving as aguest material is used as a host material, the lifetime is representedby a two-component decay curve. A short lifetime component (τ1) ofPCBBiNB is shorter and a long lifetime component (τ2) of PCBBiNB islonger than the lifetime of the sample in which PCBA1BP is used as ahost material. The lifetime of the long lifetime component (τ2) becomeslonger as the proportion of PCBBiNB increases. This is because the shortlifetime component (τ1) of PCBBiNB is the sum of an emission rate of theguest material and a transfer rate of exciton energy to the hostmaterial, and the long lifetime component (τ2) of PCBBiNB is a result ofenergy transfer from the host material to the guest material.

Thus, one feature of the light-emitting element of one embodiment of thepresent invention is that a multicomponent decay curve like the oneshown in FIG. 17 can be obtained in the case of using a host materialwhose T1 level is lower than that of a guest material.

EXPLANATION OF REFERENCE

-   10: exciton, 11: host material, 12: guest material, 101: anode, 102:    cathode, 103: EL layer, 104: light-emitting layer, 105: first    organic compound (serving as a host material), 106: second organic    compound (serving as a guest material), 201: first electrode, 202:    second electrode, 203: EL layer, 204: hole-injection layer, 205:    hole-transport layer, 206: light-emitting layer, 207:    electron-transport layer, 208: electron-injection layer, 209: first    organic compound (serving as a host material), 210: second organic    compound (serving as a guest material), 301: first electrode,    302(1): first EL layer, 302(2): second EL layer, 304: second    electrode, 305: charge-generation layer, 305(1): first    charge-generation layer, 305(2): second charge-generation layer,    501: element substrate, 502: pixel portion, 503: driver circuit    portion (source line driver circuit), 504 a, 504 b: driver circuit    portion (gate line driver circuit), 505: sealant, 506: sealing    substrate, 507: wiring, 508: FPC (flexible printed circuit), 509:    n-channel FET, 510: p-channel FET, 511: switching FET, 512: current    control FET, 513: first electrode (anode), 514: insulator, 515: EL    layer, 516: second electrode (cathode), 517: light-emitting element,    518: element layer, 1100: substrate, 1101: first electrode, 1102: EL    layer, 1103: second electrode, 1111: hole-injection layer, 1112:    hole-transport layer, 1113: light-emitting layer, 1114:    electron-transport layer, 1115: space, 2001: first substrate, 2002:    light-emitting portion, 2005 a: first sealant, 2005 b: second    sealant, 2006: second substrate, 2011: second space, 2013: first    space, 3000: light-emitting device, 3001: substrate, 3002 a:    reflective electrode, 3002 b: reflective electrode, 3002 c:    reflective electrode, 3003 a: lower electrode, 3003 b: lower    electrode, 3003 c: lower electrode, 3005 a: transparent conductive    layer, 3005 b: transparent conductive layer, 3005 c: transparent    conductive layer, 3007 a: partition wall, 3007 b: partition wall,    3007 c: partition wall, 3007 d: partition wall, 3009: hole-injection    layer, 3011 a: hole-transport layer, 3011 b: hole-transport layer,    3011 c: hole-transport layer, 3013 a: light-emitting layer, 3013 b:    light-emitting layer, 3013 c: light-emitting layer, 3015 a:    electron-transport layer, 3015 b: electron-transport layer, 3015 c:    electron-transport layer, 3017: electron-injection layer, 3019:    upper electrode, 3020 a: light-emitting element, 3020 b:    light-emitting element, 3020 c: light-emitting element, 3100:    light-emitting device, 3101: substrate, 3102 a: reflective    electrode, 3102 b: reflective electrode, 3102 c: reflective    electrode, 3103 a: lower electrode, 3103 b: lower electrode, 3103 c:    lower electrode, 3103 d: lower electrode, 3105 a: transparent    conductive layer, 3105 b: transparent conductive layer, 3107 a:    partition wall, 3107 b: partition wall, 3107 c: partition wall, 3107    d: partition wall, 3110: hole-injection and hole-transport layer,    3112: first light-emitting layer, 3114: charge-generation layer,    3116: second light-emitting layer, 3118: electron-transport and    electron-injection layer, 3119: upper electrode, 3120 a:    light-emitting element, 3120 b: light-emitting element, 3120 c:    light-emitting element, 4000: lighting device, 4001: lighting    device, 4003: substrate, 4005: substrate, 4007: light-emitting    element, 4009: electrode, 4011: electrode, 4013: lower electrode,    4014: EL layer, 4015: upper electrode, 4017: auxiliary wiring, 4019:    sealing substrate, 4021: sealant, 4023: desiccant, 4025: substrate,    4027: diffusing plate, 4100: lighting device, 4101: lighting device,    4103: sealing substrate, 4105: planarization film, 4107:    light-emitting element, 4109: electrode, 4111: electrode, 4113:    lower electrode, 4114: EL layer, 4115: upper electrode, 4117:    auxiliary wiring, 4121: sealant, 4125: substrate, 4127: diffusing    plate, 4129: barrier film, 4131: insulating layer, 4500: touch    sensor, 4510: conductive layer, 4510 a: conductive layer, 4510 b:    conductive layer, 4510 c: conductive layer, 4520: conductive layer,    4540: capacitance, 4710: electrode, 4810: insulating layer, 4820:    insulating layer, 4910: substrate, 4920: substrate, 5000: module,    5001: upper cover, 5002: lower cover, 5003: FPC, 5004: touch panel,    5005: FPC, 5006: display panel, 5007: backlight unit, 5008: light    source, 5009: frame, 5010: printed board, 5011: battery, 6001:    substrate, 6002: light-emitting element, 6003: first electrode,    6004: EL layer, 6005: second electrode, 6006: buffer layer, 6007:    third electrode, 6008: contact portion, 6100: light scattering    layer, 6101: light scatterer, 6102: air layer, 6103: high refractive    index layer, 6104: electron-injection layer, 7100: television    device, 7101: housing, 7103: display portion, 7105: stand, 7107:    display portion, 7109: operation key, 7110: remote controller, 7201:    main body, 7202: housing, 7203: display portion, 7204: keyboard,    7205: external connection port, 7206: pointing device, 7301:    housing, 7302: housing, 7303: joint portion, 7304: display portion,    7305: display portion, 7306: speaker portion, 7307: recording medium    insertion portion, 7308: LED lamp, 7309: operation key, 7310:    connection terminal, 7311: sensor, 7312: microphone, 7400: mobile    phone device, 7401: housing, 7402: display portion, 7403: operation    button, 7404: external connection port, 7405: speaker, 7406:    microphone, 8001: lighting device, 8002: lighting device, 8003:    lighting device, 8004: lighting device, 9033: clasp, 9034: display    mode switch, 9035: power supply switch, 9036: power saver switch,    9038: operation switch, 9630: housing, 9631: display portion, 9631    a: display portion, 9631 b: display portion, 9632 a: touch panel    region, 9632 b: touch panel region, 9633: solar cell, 9634:    charge/discharge control circuit, 9635: battery, 9636: DC-DC    converter, 9637: operation key, 9638: converter, 9639: button

This application is based on Japanese Patent Application serial no.2013-002296 filed with Japan Patent Office on Jan. 10, 2013, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. (canceled)
 2. A light-emitting device comprising:a light-emitting layer comprising a first organic compound and a secondorganic compound, wherein the first organic compound is a guestmaterial, wherein a T1 level of the first organic compound is equal toor higher than a T1 level of the second organic compound, whereinemission time-dependence of emission intensity of the light-emittinglayer is represented by a multicomponent decay curve, and wherein anemission time of a longest lifetime component of the multicomponentdecay curve at 25° C. is less than or equal to 15 μsec where theemission time is a time required for a value of initial emissionintensity to become 1/100.
 3. The light-emitting device according toclaim 2, wherein a difference between the T1 level of the first organiccompound and the T1 level of the second organic compound is greater thanor equal to 0 eV and less than or equal to 0.2 eV.
 4. The light-emittingdevice according to claim 2, wherein a difference between the T1 levelof the first organic compound and the T1 level of the second organiccompound is greater than or equal to 0 eV and less than or equal to 0.02eV.
 5. The light-emitting device according to claim 2, wherein the firstorganic compound is a phosphorescence compound.
 6. The light-emittingdevice according to claim 2, wherein the light-emitting device isconfigured to emit blue light.
 7. An electronic device comprising thelight-emitting device according to claim 2
 8. A lighting devicecomprising the light-emitting device according to claim
 2. 9. Alight-emitting device comprising: a light-emitting layer comprising afirst organic compound and a second organic compound, wherein the firstorganic compound is a guest material, wherein the second organiccompound is a host material, wherein a T1 level of the first organiccompound is equal to or higher than a T1 level of the second organiccompound, wherein the first organic compound and the second organiccompound are selected so that emission time-dependence of emissionintensity of the light-emitting device is represented by amulticomponent decay curve, and wherein an emission time of a longestlifetime component of the multicomponent decay curve at 25° C. is lessthan or equal to 15 μsec where the emission time is a time required fora value of initial emission intensity to become 1/100.
 10. Thelight-emitting device according to claim 9, wherein a difference betweenthe T1 level of the first organic compound and the T1 level of thesecond organic compound is greater than or equal to 0 eV and less thanor equal to 0.2 eV.
 11. The light-emitting device according to claim 9,wherein a difference between the T1 level of the first organic compoundand the T1 level of the second organic compound is greater than or equalto 0 eV and less than or equal to 0.02 eV.
 12. The light-emitting deviceaccording to claim 9, wherein the multicomponent decay curve representsat least two components derived from the first organic compound and/orthe second organic compound at the time when the value of initialemission intensity becomes 1/100.
 13. The light-emitting deviceaccording to claim 9, wherein the light-emitting device is configured toemit blue light.
 14. An electronic device comprising the light-emittingdevice according to claim
 9. 15. A lighting device comprising thelight-emitting device according to claim
 9. 16. A light-emitting devicecomprising: a light-emitting layer comprising a guest material and ahost material, wherein a T1 level of the guest material is equal to orhigher than a T1 level of the host material, wherein the guest materialand the host material are selected so that emission time-dependence ofemission intensity of the light-emitting layer is represented by amulticomponent decay curve, and wherein an emission time of a longestlifetime component of the multicomponent decay curve obtained fromphotoluminescence at 25° C. is less than or equal to 15 μsec where theemission time is a time required for a value of initial emissionintensity to become 1/100.
 17. The light-emitting device according toclaim 16, wherein a difference between the T1 level of the guestmaterial and the T1 level of the host material is greater than or equalto 0 eV and less than or equal to 0.2 eV.
 18. The light-emitting deviceaccording to claim 16, wherein a difference between the T1 level of theguest material and the T1 level of the host material is greater than orequal to 0 eV and less than or equal to 0.02 eV.
 19. The light-emittingdevice according to claim 16, wherein the guest material is aphosphorescence compound.
 20. The light-emitting device according toclaim 16, wherein the multicomponent decay curve represents at least twocomponents derived from the guest material at the time when the value ofinitial emission intensity becomes 1/100.
 21. The light-emitting deviceaccording to claim 16, wherein the light-emitting device is configuredto emit blue light.
 22. An electronic device comprising thelight-emitting device according to claim
 16. 23. A lighting devicecomprising the light-emitting device according to claim 16.